GNU make

The GNU make utility automatically determines which pieces of a large program need to be recompiled, and issues the commands to recompile them.

This edition of the GNU Make Manual, last updated 08 July 2002, documents GNU make Version 3.80.

This manual describes make and contains the following chapters:

1. Overview of make
2. An Introduction to Makefiles		An introduction to make.
3. Writing Makefiles		Makefiles tell make what to do.
4. Writing Rules		Rules describe when a file must be remade.
5. Writing the Commands in Rules		Commands say how to remake a file.
6. How to Use Variables		You can use variables to avoid repetition.
7. Conditional Parts of Makefiles		Use or ignore parts of the makefile based on the values of variables.
8. Functions for Transforming Text		Many powerful ways to manipulate text.
9. How to Run make		How to invoke make on the command line.
10. Using Implicit Rules		Use implicit rules to treat many files alike, based on their file names.
11. Using make to Update Archive Files		How make can update library archives.
12. Features of GNU make		Features GNU make has over other makes.
13. Incompatibilities and Missing Features		What GNU make lacks from other makes.
14. Makefile Conventions		Conventions for writing makefiles for GNU programs.
A. Quick Reference		A quick reference for experienced users.
B. Errors Generated by Make		A list of common errors generated by make.
C. Complex Makefile Example		A real example of a straightforward, but nontrivial, makefile.

D. GNU Free Documentation License		License for copying this manual
Index of Concepts
Index of Functions, Variables, & Directives

 -- The Detailed Node Listing ---

Overview of make

Preparing and Running Make
1.1 How to Read This Manual		On Reading this Text
1.2 Problems and Bugs

An Introduction to Makefiles

2.1 What a Rule Looks Like		What a rule looks like.
2.2 A Simple Makefile
2.3 How make Processes a Makefile		How make Processes This Makefile
2.4 Variables Make Makefiles Simpler
2.5 Letting make Deduce the Commands
2.6 Another Style of Makefile
2.7 Rules for Cleaning the Directory

Writing Makefiles

3.1 What Makefiles Contain		What makefiles contain.
3.2 What Name to Give Your Makefile		How to name your makefile.
3.3 Including Other Makefiles		How one makefile can use another makefile.
3.4 The Variable MAKEFILES		The environment can specify extra makefiles.
3.5 The Variable MAKEFILE_LIST		Discover which makefiles have been read.
3.6 Other Special Variables		Other special variables.
3.7 How Makefiles Are Remade		How makefiles get remade.
3.8 Overriding Part of Another Makefile		How to override part of one makefile with another makefile.
3.9 How make Reads a Makefile		How makefiles are parsed.

Writing Rules

4.1 Rule Example		An example explained.
4.2 Rule Syntax		General syntax explained.
4.3 Types of Prerequisites		There are two types of prerequisites.
4.4 Using Wildcard Characters in File Names		Using wildcard characters such as `*'.
4.5 Searching Directories for Prerequisites		Searching other directories for source files.
4.6 Phony Targets		Using a target that is not a real file's name.
4.7 Rules without Commands or Prerequisites		You can use a target without commands or prerequisites to mark other targets as phony.
4.8 Empty Target Files to Record Events		When only the date matters and the files are empty.
4.9 Special Built-in Target Names		Targets with special built-in meanings.
4.10 Multiple Targets in a Rule		When to make use of several targets in a rule.
4.11 Multiple Rules for One Target		How to use several rules with the same target.
4.12 Static Pattern Rules		Static pattern rules apply to multiple targets and can vary the prerequisites according to the target name.
4.13 Double-Colon Rules		How to use a special kind of rule to allow several independent rules for one target.
4.14 Generating Prerequisites Automatically		How to automatically generate rules giving prerequisites from source files themselves.

Using Wildcard Characters in File Names

4.4.1 Wildcard Examples		Several examples
4.4.2 Pitfalls of Using Wildcards		Problems to avoid.
4.4.3 The Function wildcard		How to cause wildcard expansion where it does not normally take place.

Searching Directories for Prerequisites

4.5.1 VPATH: Search Path for All Prerequisites		Specifying a search path that applies to every prerequisite.
4.5.2 The vpath Directive		Specifying a search path for a specified class of names.
4.5.3 How Directory Searches are Performed		When and how search paths are applied.
4.5.4 Writing Shell Commands with Directory Search		How to write shell commands that work together with search paths.
4.5.5 Directory Search and Implicit Rules		How search paths affect implicit rules.
4.5.6 Directory Search for Link Libraries		Directory search for link libraries.

Static Pattern Rules

4.12.1 Syntax of Static Pattern Rules		The syntax of static pattern rules.
4.12.2 Static Pattern Rules versus Implicit Rules		When are they better than implicit rules?

Writing the Commands in Rules

5.1 Command Echoing		How to control when commands are echoed.
5.2 Command Execution		How commands are executed.
5.3 Parallel Execution		How commands can be executed in parallel.
5.4 Errors in Commands		What happens after a command execution error.
5.5 Interrupting or Killing make		What happens when a command is interrupted.
5.6 Recursive Use of make		Invoking make from makefiles.
5.7 Defining Canned Command Sequences		Defining canned sequences of commands.
5.8 Using Empty Commands		Defining useful, do-nothing commands.

Recursive Use of make

5.6.1 How the MAKE Variable Works		The special effects of using `$(MAKE)'.
5.6.2 Communicating Variables to a Sub-make		How to communicate variables to a sub-make.
5.6.3 Communicating Options to a Sub-make		How to communicate options to a sub-make.
5.6.4 The `--print-directory' Option		How the `-w' or `--print-directory' option helps debug use of recursive make commands.

How to Use Variables

6.1 Basics of Variable References		How to use the value of a variable.
6.2 The Two Flavors of Variables		Variables come in two flavors.
6.3 Advanced Features for Reference to Variables		Advanced features for referencing a variable.
6.4 How Variables Get Their Values		All the ways variables get their values.
6.5 Setting Variables		How to set a variable in the makefile.
6.6 Appending More Text to Variables		How to append more text to the old value of a variable.
6.7 The override Directive		How to set a variable in the makefile even if the user has set it with a command argument.
6.8 Defining Variables Verbatim		An alternate way to set a variable to a verbatim string.
6.9 Variables from the Environment		Variable values can come from the environment.
6.10 Target-specific Variable Values		Variable values can be defined on a per-target basis.
6.11 Pattern-specific Variable Values		Target-specific variable values can be applied to a group of targets that match a pattern.

Advanced Features for Reference to Variables

6.3.1 Substitution References		Referencing a variable with substitutions on the value.
6.3.2 Computed Variable Names		Computing the name of the variable to refer to.

Conditional Parts of Makefiles

7.1 Example of a Conditional		Example of a conditional
7.2 Syntax of Conditionals		The syntax of conditionals.
7.3 Conditionals that Test Flags		Conditionals that test flags.

Functions for Transforming Text

8.1 Function Call Syntax		How to write a function call.
8.2 Functions for String Substitution and Analysis		General-purpose text manipulation functions.
8.3 Functions for File Names		Functions for manipulating file names.
8.4 The foreach Function		Repeat some text with controlled variation.
8.5 The if Function		Conditionally expand a value.
8.6 The call Function		Expand a user-defined function.
8.7 The value Function		Return the un-expanded value of a variable.
8.8 The eval Function		Evaluate the arguments as makefile syntax.
8.9 The origin Function		Find where a variable got its value.
8.10 The shell Function		Substitute the output of a shell command.
8.11 Functions That Control Make		Functions that control how make runs.

How to Run make

9.1 Arguments to Specify the Makefile		How to specify which makefile to use.
9.2 Arguments to Specify the Goals		How to use goal arguments to specify which parts of the makefile to use.
9.3 Instead of Executing the Commands		How to use mode flags to specify what kind of thing to do with the commands in the makefile other than simply execute them.
9.4 Avoiding Recompilation of Some Files		How to avoid recompiling certain files.
9.5 Overriding Variables		How to override a variable to specify an alternate compiler and other things.
9.6 Testing the Compilation of a Program		How to proceed past some errors, to test compilation.
9.7 Summary of Options

Using Implicit Rules

10.1 Using Implicit Rules		How to use an existing implicit rule to get the commands for updating a file.
10.2 Catalogue of Implicit Rules		A list of built-in implicit rules.
10.3 Variables Used by Implicit Rules		How to change what predefined rules do.
10.4 Chains of Implicit Rules		How to use a chain of implicit rules.
10.5 Defining and Redefining Pattern Rules		How to define new implicit rules.
10.6 Defining Last-Resort Default Rules		How to defining commands for rules which cannot find any.
10.7 Old-Fashioned Suffix Rules		The old-fashioned style of implicit rule.
10.8 Implicit Rule Search Algorithm		The precise algorithm for applying implicit rules.

Defining and Redefining Pattern Rules

10.5.1 Introduction to Pattern Rules		An introduction to pattern rules.
10.5.2 Pattern Rule Examples		Examples of pattern rules.
10.5.3 Automatic Variables		How to use automatic variables in the commands of implicit rules.
10.5.4 How Patterns Match		How patterns match.
10.5.5 Match-Anything Pattern Rules		Precautions you should take prior to defining rules that can match any target file whatever.
10.5.6 Canceling Implicit Rules		How to override or cancel built-in rules.

Using make to Update Archive Files

11.1 Archive Members as Targets		Archive members as targets.
11.2 Implicit Rule for Archive Member Targets		The implicit rule for archive member targets.
11.3 Dangers When Using Archives		Dangers to watch out for when using archives.
11.4 Suffix Rules for Archive Files		You can write a special kind of suffix rule for updating archives.

Implicit Rule for Archive Member Targets

11.2.1 Updating Archive Symbol Directories

How to update archive symbol directories.

Makefile Conventions

14.1 General Conventions for Makefiles
14.2 Utilities in Makefiles
14.3 Variables for Specifying Commands
14.4 Variables for Installation Directories
14.5 Standard Targets for Users
14.6 Install Command Categories		Three categories of commands in the `install'

Copying This Manual

1. Overview of make

The make utility automatically determines which pieces of a large program need to be recompiled, and issues commands to recompile them. This manual describes GNU make, which was implemented by Richard Stallman and Roland McGrath. Development since Version 3.76 has been handled by Paul Smith.

GNU make conforms to section 6.2 of IEEE Standard 1003.2-1992 (POSIX.2).

Our examples show C programs, since they are most common, but you can use make with any programming language whose compiler can be run with a shell command. Indeed, make is not limited to programs. You can use it to describe any task where some files must be updated automatically from others whenever the others change.

Preparing and Running Make
1.1 How to Read This Manual		On Reading this Text
1.2 Problems and Bugs

Preparing and Running Make

To prepare to use make, you must write a file called the makefile that describes the relationships among files in your program and provides commands for updating each file. In a program, typically, the executable file is updated from object files, which are in turn made by compiling source files.

Once a suitable makefile exists, each time you change some source files, this simple shell command:

make

suffices to perform all necessary recompilations. The make program uses the makefile data base and the last-modification times of the files to decide which of the files need to be updated. For each of those files, it issues the commands recorded in the data base.

You can provide command line arguments to make to control which files should be recompiled, or how. See section How to Run make.

1.1 How to Read This Manual

If you are new to make, or are looking for a general introduction, read the first few sections of each chapter, skipping the later sections. In each chapter, the first few sections contain introductory or general information and the later sections contain specialized or technical information. The exception is the second chapter, An Introduction to Makefiles, all of which is introductory.

If you are familiar with other make programs, see Features of GNU make, which lists the enhancements GNU make has, and Incompatibilities and Missing Features, which explains the few things GNU make lacks that others have.

For a quick summary, see 9.7 Summary of Options, A. Quick Reference, and 4.9 Special Built-in Target Names.

1.2 Problems and Bugs

If you have problems with GNU make or think you've found a bug, please report it to the developers; we cannot promise to do anything but we might well want to fix it.

Before reporting a bug, make sure you've actually found a real bug. Carefully reread the documentation and see if it really says you can do what you're trying to do. If it's not clear whether you should be able to do something or not, report that too; it's a bug in the documentation!

Before reporting a bug or trying to fix it yourself, try to isolate it to the smallest possible makefile that reproduces the problem. Then send us the makefile and the exact results make gave you, including any error or warning messages. Please don't paraphrase these messages: it's best to cut and paste them into your report. When generating this small makefile, be sure to not use any non-free or unusual tools in your commands: you can almost always emulate what such a tool would do with simple shell commands. Finally, be sure to explain what you expected to occur; this will help us decide whether the problem was really in the documentation.

Once you have a precise problem you can report it in one of two ways. Either send electronic mail to:

bug-make@gnu.org

or use our Web-based project management tool, at:

http://savannah.gnu.org/projects/make/

In addition to the information above, please be careful to include the version number of make you are using. You can get this information with the command `make --version'. Be sure also to include the type of machine and operating system you are using. One way to obtain this information is by looking at the final lines of output from the command `make --help'.

2. An Introduction to Makefiles

You need a file called a makefile to tell make what to do. Most often, the makefile tells make how to compile and link a program.

In this chapter, we will discuss a simple makefile that describes how to compile and link a text editor which consists of eight C source files and three header files. The makefile can also tell make how to run miscellaneous commands when explicitly asked (for example, to remove certain files as a clean-up operation). To see a more complex example of a makefile, see C. Complex Makefile Example.

When make recompiles the editor, each changed C source file must be recompiled. If a header file has changed, each C source file that includes the header file must be recompiled to be safe. Each compilation produces an object file corresponding to the source file. Finally, if any source file has been recompiled, all the object files, whether newly made or saved from previous compilations, must be linked together to produce the new executable editor.

2.1 What a Rule Looks Like		What a rule looks like.
2.2 A Simple Makefile
2.3 How make Processes a Makefile		How make Processes This Makefile
2.4 Variables Make Makefiles Simpler
2.5 Letting make Deduce the Commands
2.6 Another Style of Makefile
2.7 Rules for Cleaning the Directory

2.1 What a Rule Looks Like

A simple makefile consists of "rules" with the following shape:

target ... : prerequisites ... command ... ...

A target is usually the name of a file that is generated by a program; examples of targets are executable or object files. A target can also be the name of an action to carry out, such as `clean' (see section 4.6 Phony Targets).

A prerequisite is a file that is used as input to create the target. A target often depends on several files.

A command is an action that make carries out. A rule may have more than one command, each on its own line. Please note: you need to put a tab character at the beginning of every command line! This is an obscurity that catches the unwary.

Usually a command is in a rule with prerequisites and serves to create a target file if any of the prerequisites change. However, the rule that specifies commands for the target need not have prerequisites. For example, the rule containing the delete command associated with the target `clean' does not have prerequisites.

A rule, then, explains how and when to remake certain files which are the targets of the particular rule. make carries out the commands on the prerequisites to create or update the target. A rule can also explain how and when to carry out an action. See section Writing Rules.

A makefile may contain other text besides rules, but a simple makefile need only contain rules. Rules may look somewhat more complicated than shown in this template, but all fit the pattern more or less.

2.2 A Simple Makefile

Here is a straightforward makefile that describes the way an executable file called edit depends on eight object files which, in turn, depend on eight C source and three header files.

In this example, all the C files include `defs.h', but only those defining editing commands include `command.h', and only low level files that change the editor buffer include `buffer.h'.

edit : main.o kbd.o command.o display.o \ insert.o search.o files.o utils.o cc -o edit main.o kbd.o command.o display.o \ insert.o search.o files.o utils.o main.o : main.c defs.h cc -c main.c kbd.o : kbd.c defs.h command.h cc -c kbd.c command.o : command.c defs.h command.h cc -c command.c display.o : display.c defs.h buffer.h cc -c display.c insert.o : insert.c defs.h buffer.h cc -c insert.c search.o : search.c defs.h buffer.h cc -c search.c files.o : files.c defs.h buffer.h command.h cc -c files.c utils.o : utils.c defs.h cc -c utils.c clean : rm edit main.o kbd.o command.o display.o \ insert.o search.o files.o utils.o

We split each long line into two lines using backslash-newline; this is like using one long line, but is easier to read.

To use this makefile to create the executable file called `edit', type:

make

To use this makefile to delete the executable file and all the object files from the directory, type:

make clean

In the example makefile, the targets include the executable file `edit', and the object files `main.o' and `kbd.o'. The prerequisites are files such as `main.c' and `defs.h'. In fact, each `.o' file is both a target and a prerequisite. Commands include `cc -c main.c' and `cc -c kbd.c'.

When a target is a file, it needs to be recompiled or relinked if any of its prerequisites change. In addition, any prerequisites that are themselves automatically generated should be updated first. In this example, `edit' depends on each of the eight object files; the object file `main.o' depends on the source file `main.c' and on the header file `defs.h'.

A shell command follows each line that contains a target and prerequisites. These shell commands say how to update the target file. A tab character must come at the beginning of every command line to distinguish commands lines from other lines in the makefile. (Bear in mind that make does not know anything about how the commands work. It is up to you to supply commands that will update the target file properly. All make does is execute the commands in the rule you have specified when the target file needs to be updated.)

The target `clean' is not a file, but merely the name of an action. Since you normally do not want to carry out the actions in this rule, `clean' is not a prerequisite of any other rule. Consequently, make never does anything with it unless you tell it specifically. Note that this rule not only is not a prerequisite, it also does not have any prerequisites, so the only purpose of the rule is to run the specified commands. Targets that do not refer to files but are just actions are called phony targets. See section 4.6 Phony Targets, for information about this kind of target. See section Errors in Commands, to see how to cause make to ignore errors from rm or any other command.

2.3 How make Processes a Makefile

By default, make starts with the first target (not targets whose names start with `.'). This is called the default goal. (Goals are the targets that make strives ultimately to update. See section Arguments to Specify the Goals.)

In the simple example of the previous section, the default goal is to update the executable program `edit'; therefore, we put that rule first.

Thus, when you give the command:

make

make reads the makefile in the current directory and begins by processing the first rule. In the example, this rule is for relinking `edit'; but before make can fully process this rule, it must process the rules for the files that `edit' depends on, which in this case are the object files. Each of these files is processed according to its own rule. These rules say to update each `.o' file by compiling its source file. The recompilation must be done if the source file, or any of the header files named as prerequisites, is more recent than the object file, or if the object file does not exist.

The other rules are processed because their targets appear as prerequisites of the goal. If some other rule is not depended on by the goal (or anything it depends on, etc.), that rule is not processed, unless you tell make to do so (with a command such as make clean).

Before recompiling an object file, make considers updating its prerequisites, the source file and header files. This makefile does not specify anything to be done for them--the `.c' and `.h' files are not the targets of any rules--so make does nothing for these files. But make would update automatically generated C programs, such as those made by Bison or Yacc, by their own rules at this time.

After recompiling whichever object files need it, make decides whether to relink `edit'. This must be done if the file `edit' does not exist, or if any of the object files are newer than it. If an object file was just recompiled, it is now newer than `edit', so `edit' is relinked.

Thus, if we change the file `insert.c' and run make, make will compile that file to update `insert.o', and then link `edit'. If we change the file `command.h' and run make, make will recompile the object files `kbd.o', `command.o' and `files.o' and then link the file `edit'.

2.4 Variables Make Makefiles Simpler

In our example, we had to list all the object files twice in the rule for `edit' (repeated here):

edit : main.o kbd.o command.o display.o \ insert.o search.o files.o utils.o cc -o edit main.o kbd.o command.o display.o \ insert.o search.o files.o utils.o

Such duplication is error-prone; if a new object file is added to the system, we might add it to one list and forget the other. We can eliminate the risk and simplify the makefile by using a variable. Variables allow a text string to be defined once and substituted in multiple places later (see section How to Use Variables).

It is standard practice for every makefile to have a variable named objects, OBJECTS, objs, OBJS, obj, or OBJ which is a list of all object file names. We would define such a variable objects with a line like this in the makefile:

objects = main.o kbd.o command.o display.o \ insert.o search.o files.o utils.o

Then, each place we want to put a list of the object file names, we can substitute the variable's value by writing `$(objects)' (see section How to Use Variables).

Here is how the complete simple makefile looks when you use a variable for the object files:

objects = main.o kbd.o command.o display.o \ insert.o search.o files.o utils.o edit : $(objects) cc -o edit $(objects) main.o : main.c defs.h cc -c main.c kbd.o : kbd.c defs.h command.h cc -c kbd.c command.o : command.c defs.h command.h cc -c command.c display.o : display.c defs.h buffer.h cc -c display.c insert.o : insert.c defs.h buffer.h cc -c insert.c search.o : search.c defs.h buffer.h cc -c search.c files.o : files.c defs.h buffer.h command.h cc -c files.c utils.o : utils.c defs.h cc -c utils.c clean : rm edit $(objects)

2.5 Letting make Deduce the Commands

It is not necessary to spell out the commands for compiling the individual C source files, because make can figure them out: it has an implicit rule for updating a `.o' file from a correspondingly named `.c' file using a `cc -c' command. For example, it will use the command `cc -c main.c -o main.o' to compile `main.c' into `main.o'. We can therefore omit the commands from the rules for the object files. See section Using Implicit Rules.

When a `.c' file is used automatically in this way, it is also automatically added to the list of prerequisites. We can therefore omit the `.c' files from the prerequisites, provided we omit the commands.

Here is the entire example, with both of these changes, and a variable objects as suggested above:

objects = main.o kbd.o command.o display.o \ insert.o search.o files.o utils.o edit : $(objects) cc -o edit $(objects) main.o : defs.h kbd.o : defs.h command.h command.o : defs.h command.h display.o : defs.h buffer.h insert.o : defs.h buffer.h search.o : defs.h buffer.h files.o : defs.h buffer.h command.h utils.o : defs.h .PHONY : clean clean : rm edit $(objects)

This is how we would write the makefile in actual practice. (The complications associated with `clean' are described elsewhere. See 4.6 Phony Targets, and Errors in Commands.)

Because implicit rules are so convenient, they are important. You will see them used frequently.

2.6 Another Style of Makefile

When the objects of a makefile are created only by implicit rules, an alternative style of makefile is possible. In this style of makefile, you group entries by their prerequisites instead of by their targets. Here is what one looks like:

objects = main.o kbd.o command.o display.o \ insert.o search.o files.o utils.o edit : $(objects) cc -o edit $(objects) $(objects) : defs.h kbd.o command.o files.o : command.h display.o insert.o search.o files.o : buffer.h

Here `defs.h' is given as a prerequisite of all the object files; `command.h' and `buffer.h' are prerequisites of the specific object files listed for them.

Whether this is better is a matter of taste: it is more compact, but some people dislike it because they find it clearer to put all the information about each target in one place.

2.7 Rules for Cleaning the Directory

Compiling a program is not the only thing you might want to write rules for. Makefiles commonly tell how to do a few other things besides compiling a program: for example, how to delete all the object files and executables so that the directory is `clean'.

Here is how we could write a make rule for cleaning our example editor:

clean: rm edit $(objects)

In practice, we might want to write the rule in a somewhat more complicated manner to handle unanticipated situations. We would do this:

.PHONY : clean clean : -rm edit $(objects)

This prevents make from getting confused by an actual file called `clean' and causes it to continue in spite of errors from rm. (See 4.6 Phony Targets, and Errors in Commands.)

A rule such as this should not be placed at the beginning of the makefile, because we do not want it to run by default! Thus, in the example makefile, we want the rule for edit, which recompiles the editor, to remain the default goal.

Since clean is not a prerequisite of edit, this rule will not run at all if we give the command `make' with no arguments. In order to make the rule run, we have to type `make clean'. See section How to Run make.

3. Writing Makefiles

The information that tells make how to recompile a system comes from reading a data base called the makefile.

3.1 What Makefiles Contain		What makefiles contain.
3.2 What Name to Give Your Makefile		How to name your makefile.
3.3 Including Other Makefiles		How one makefile can use another makefile.
3.4 The Variable MAKEFILES		The environment can specify extra makefiles.
3.5 The Variable MAKEFILE_LIST		Discover which makefiles have been read.
3.6 Other Special Variables		Other special variables.
3.7 How Makefiles Are Remade		How makefiles get remade.
3.8 Overriding Part of Another Makefile		How to override part of one makefile with another makefile.
3.9 How make Reads a Makefile		How makefiles are parsed.

3.1 What Makefiles Contain

Makefiles contain five kinds of things: explicit rules, implicit rules, variable definitions, directives, and comments. Rules, variables, and directives are described at length in later chapters.

• An explicit rule says when and how to remake one or more files, called the rule's targets. It lists the other files that the targets depend on, call the prerequisites of the target, and may also give commands to use to create or update the targets. See section Writing Rules.

• An implicit rule says when and how to remake a class of files based on their names. It describes how a target may depend on a file with a name similar to the target and gives commands to create or update such a target. See section Using Implicit Rules.

• A variable definition is a line that specifies a text string value for a variable that can be substituted into the text later. The simple makefile example shows a variable definition for objects as a list of all object files (see section Variables Make Makefiles Simpler).

• A directive is a command for make to do something special while reading the makefile. These include:
  • Reading another makefile (see section Including Other Makefiles).

  • Deciding (based on the values of variables) whether to use or ignore a part of the makefile (see section Conditional Parts of Makefiles).

  • Defining a variable from a verbatim string containing multiple lines (see section Defining Variables Verbatim).

• `#' in a line of a makefile starts a comment. It and the rest of the line are ignored, except that a trailing backslash not escaped by another backslash will continue the comment across multiple lines. A line containing just a comment (with perhaps spaces before it) is effectively blank, and is ignored. If you want a literal #, escape it with a backslash (e.g., \#). Comments may appear on any line in the makefile, although they are treated specially in certain situations.

  Within a command script (if the line begins with a TAB character) the entire line is passed to the shell, just as with any other line that begins with a TAB. The shell decides how to interpret the text: whether or not this is a comment is up to the shell.

  Within a define directive, comments are not ignored during the definition of the variable, but rather kept intact in the value of the variable. When the variable is expanded they will either be treated as make comments or as command script text, depending on the context in which the variable is evaluated.

3.2 What Name to Give Your Makefile

By default, when make looks for the makefile, it tries the following names, in order: `GNUmakefile', `makefile' and `Makefile'.

Normally you should call your makefile either `makefile' or `Makefile'. (We recommend `Makefile' because it appears prominently near the beginning of a directory listing, right near other important files such as `README'.) The first name checked, `GNUmakefile', is not recommended for most makefiles. You should use this name if you have a makefile that is specific to GNU make, and will not be understood by other versions of make. Other make programs look for `makefile' and `Makefile', but not `GNUmakefile'.

If make finds none of these names, it does not use any makefile. Then you must specify a goal with a command argument, and make will attempt to figure out how to remake it using only its built-in implicit rules. See section Using Implicit Rules.

If you want to use a nonstandard name for your makefile, you can specify the makefile name with the `-f' or `--file' option. The arguments `-f name' or `--file=name' tell make to read the file name as the makefile. If you use more than one `-f' or `--file' option, you can specify several makefiles. All the makefiles are effectively concatenated in the order specified. The default makefile names `GNUmakefile', `makefile' and `Makefile' are not checked automatically if you specify `-f' or `--file'.

3.3 Including Other Makefiles

The include directive tells make to suspend reading the current makefile and read one or more other makefiles before continuing. The directive is a line in the makefile that looks like this:

include filenames...

filenames can contain shell file name patterns.

Extra spaces are allowed and ignored at the beginning of the line, but a tab is not allowed. (If the line begins with a tab, it will be considered a command line.) Whitespace is required between include and the file names, and between file names; extra whitespace is ignored there and at the end of the directive. A comment starting with `#' is allowed at the end of the line. If the file names contain any variable or function references, they are expanded. See section How to Use Variables.

For example, if you have three `.mk' files, `a.mk', `b.mk', and `c.mk', and $(bar) expands to bish bash, then the following expression

include foo *.mk $(bar)

is equivalent to

include foo a.mk b.mk c.mk bish bash

When make processes an include directive, it suspends reading of the containing makefile and reads from each listed file in turn. When that is finished, make resumes reading the makefile in which the directive appears.

One occasion for using include directives is when several programs, handled by individual makefiles in various directories, need to use a common set of variable definitions (see section Setting Variables) or pattern rules (see section Defining and Redefining Pattern Rules).

Another such occasion is when you want to generate prerequisites from source files automatically; the prerequisites can be put in a file that is included by the main makefile. This practice is generally cleaner than that of somehow appending the prerequisites to the end of the main makefile as has been traditionally done with other versions of make. See section 4.14 Generating Prerequisites Automatically.

If the specified name does not start with a slash, and the file is not found in the current directory, several other directories are searched. First, any directories you have specified with the `-I' or `--include-dir' option are searched (see section Summary of Options). Then the following directories (if they exist) are searched, in this order: `prefix/include' (normally `/usr/local/include' (1)) `/usr/gnu/include', `/usr/local/include', `/usr/include'.

If an included makefile cannot be found in any of these directories, a warning message is generated, but it is not an immediately fatal error; processing of the makefile containing the include continues. Once it has finished reading makefiles, make will try to remake any that are out of date or don't exist. See section How Makefiles Are Remade. Only after it has tried to find a way to remake a makefile and failed, will make diagnose the missing makefile as a fatal error.

If you want make to simply ignore a makefile which does not exist and cannot be remade, with no error message, use the -include directive instead of include, like this:

-include filenames...

This acts like include in every way except that there is no error (not even a warning) if any of the filenames do not exist. For compatibility with some other make implementations, sinclude is another name for -include.

3.4 The Variable MAKEFILES

If the environment variable MAKEFILES is defined, make considers its value as a list of names (separated by whitespace) of additional makefiles to be read before the others. This works much like the include directive: various directories are searched for those files (see section Including Other Makefiles). In addition, the default goal is never taken from one of these makefiles and it is not an error if the files listed in MAKEFILES are not found.

The main use of MAKEFILES is in communication between recursive invocations of make (see section Recursive Use of make). It usually is not desirable to set the environment variable before a top-level invocation of make, because it is usually better not to mess with a makefile from outside. However, if you are running make without a specific makefile, a makefile in MAKEFILES can do useful things to help the built-in implicit rules work better, such as defining search paths (see section 4.5 Searching Directories for Prerequisites).

Some users are tempted to set MAKEFILES in the environment automatically on login, and program makefiles to expect this to be done. This is a very bad idea, because such makefiles will fail to work if run by anyone else. It is much better to write explicit include directives in the makefiles. See section Including Other Makefiles.

3.5 The Variable MAKEFILE_LIST

As make reads various makefiles, including any obtained from the MAKEFILES variable, the command line, the default files, or from include directives, their names will be automatically appended to the MAKEFILE_LIST variable. They are added right before make begins to parse them.

This means that if the first thing a makefile does is examine the last word in this variable, it will be the name of the current makefile. Once the current makefile has used include, however, the last word will be the just-included makefile.

If a makefile named Makefile has this content:

name1 := $(word $(words $(MAKEFILE_LIST)),$(MAKEFILE_LIST)) include inc.mk name2 := $(word $(words $(MAKEFILE_LIST)),$(MAKEFILE_LIST)) all: @echo name1 = $(name1) @echo name2 = $(name2)

then you would expect to see this output:

name1 = Makefile name2 = inc.mk

See section 8.2 Functions for String Substitution and Analysis, for more information on the word and words functions used above. See section The Two Flavors of Variables, for more information on simply-expanded (:=) variable definitions.

3.6 Other Special Variables

GNU make also supports a special variable. Note that any value you assign to this variable will be ignored; it will always return its special value.

The first special variable is .VARIABLES. When expanded, the value consists of a list of the names of all global variables defined in all makefiles read up until that point. This includes variables which have empty values, as well as built-in variables (see section Variables Used by Implicit Rules), but does not include any variables which are only defined in a target-specific context.

3.7 How Makefiles Are Remade

Sometimes makefiles can be remade from other files, such as RCS or SCCS files. If a makefile can be remade from other files, you probably want make to get an up-to-date version of the makefile to read in.

To this end, after reading in all makefiles, make will consider each as a goal target and attempt to update it. If a makefile has a rule which says how to update it (found either in that very makefile or in another one) or if an implicit rule applies to it (see section Using Implicit Rules), it will be updated if necessary. After all makefiles have been checked, if any have actually been changed, make starts with a clean slate and reads all the makefiles over again. (It will also attempt to update each of them over again, but normally this will not change them again, since they are already up to date.)

If you know that one or more of your makefiles cannot be remade and you want to keep make from performing an implicit rule search on them, perhaps for efficiency reasons, you can use any normal method of preventing implicit rule lookup to do so. For example, you can write an explicit rule with the makefile as the target, and an empty command string (see section Using Empty Commands).

If the makefiles specify a double-colon rule to remake a file with commands but no prerequisites, that file will always be remade (see section 4.13 Double-Colon Rules). In the case of makefiles, a makefile that has a double-colon rule with commands but no prerequisites will be remade every time make is run, and then again after make starts over and reads the makefiles in again. This would cause an infinite loop: make would constantly remake the makefile, and never do anything else. So, to avoid this, make will not attempt to remake makefiles which are specified as targets of a double-colon rule with commands but no prerequisites.

If you do not specify any makefiles to be read with `-f' or `--file' options, make will try the default makefile names; see section What Name to Give Your Makefile. Unlike makefiles explicitly requested with `-f' or `--file' options, make is not certain that these makefiles should exist. However, if a default makefile does not exist but can be created by running make rules, you probably want the rules to be run so that the makefile can be used.

Therefore, if none of the default makefiles exists, make will try to make each of them in the same order in which they are searched for (see section What Name to Give Your Makefile) until it succeeds in making one, or it runs out of names to try. Note that it is not an error if make cannot find or make any makefile; a makefile is not always necessary.

When you use the `-t' or `--touch' option (see section Instead of Executing the Commands), you would not want to use an out-of-date makefile to decide which targets to touch. So the `-t' option has no effect on updating makefiles; they are really updated even if `-t' is specified. Likewise, `-q' (or `--question') and `-n' (or `--just-print') do not prevent updating of makefiles, because an out-of-date makefile would result in the wrong output for other targets. Thus, `make -f mfile -n foo' will update `mfile', read it in, and then print the commands to update `foo' and its prerequisites without running them. The commands printed for `foo' will be those specified in the updated contents of `mfile'.

However, on occasion you might actually wish to prevent updating of even the makefiles. You can do this by specifying the makefiles as goals in the command line as well as specifying them as makefiles. When the makefile name is specified explicitly as a goal, the options `-t' and so on do apply to them.

Thus, `make -f mfile -n mfile foo' would read the makefile `mfile', print the commands needed to update it without actually running them, and then print the commands needed to update `foo' without running them. The commands for `foo' will be those specified by the existing contents of `mfile'.

3.8 Overriding Part of Another Makefile

Sometimes it is useful to have a makefile that is mostly just like another makefile. You can often use the `include' directive to include one in the other, and add more targets or variable definitions. However, if the two makefiles give different commands for the same target, make will not let you just do this. But there is another way.

In the containing makefile (the one that wants to include the other), you can use a match-anything pattern rule to say that to remake any target that cannot be made from the information in the containing makefile, make should look in another makefile. See section 10.5 Defining and Redefining Pattern Rules, for more information on pattern rules.

For example, if you have a makefile called `Makefile' that says how to make the target `foo' (and other targets), you can write a makefile called `GNUmakefile' that contains:

foo: frobnicate > foo %: force @$(MAKE) -f Makefile $@ force: ;

If you say `make foo', make will find `GNUmakefile', read it, and see that to make `foo', it needs to run the command `frobnicate > foo'. If you say `make bar', make will find no way to make `bar' in `GNUmakefile', so it will use the commands from the pattern rule: `make -f Makefile bar'. If `Makefile' provides a rule for updating `bar', make will apply the rule. And likewise for any other target that `GNUmakefile' does not say how to make.

The way this works is that the pattern rule has a pattern of just `%', so it matches any target whatever. The rule specifies a prerequisite `force', to guarantee that the commands will be run even if the target file already exists. We give `force' target empty commands to prevent make from searching for an implicit rule to build it--otherwise it would apply the same match-anything rule to `force' itself and create a prerequisite loop!

3.9 How make Reads a Makefile

GNU make does its work in two distinct phases. During the first phase it reads all the makefiles, included makefiles, etc. and internalizes all the variables and their values, implicit and explicit rules, and constructs a dependency graph of all the targets and their prerequisites. During the second phase, make uses these internal structures to determine what targets will need to be rebuilt and to invoke the rules necessary to do so.

It's important to understand this two-phase approach because it has a direct impact on how variable and function expansion happens; this is often a source of some confusion when writing makefiles. Here we will present a summary of the phases in which expansion happens for different constructs within the makefile. We say that expansion is immediate if it happens during the first phase: in this case make will expand any variables or functions in that section of a construct as the makefile is parsed. We say that expansion is deferred if expansion is not performed immediately. Expansion of deferred construct is not performed until either the construct appears later in an immediate context, or until the second phase.

You may not be familiar with some of these constructs yet. You can reference this section as you become familiar with them, in later chapters.

Variable Assignment

Variable definitions are parsed as follows:

immediate = deferred immediate ?= deferred immediate := immediate immediate += deferred or immediate define immediate deferred endef

For the append operator, `+=', the right-hand side is considered immediate if the variable was previously set as a simple variable (`:='), and deferred otherwise.

Conditional Statements

All instances of conditional syntax are parsed immediately, in their entirety; this includes the ifdef, ifeq, ifndef, and ifneq forms.

Rule Definition

A rule is always expanded the same way, regardless of the form:

immediate : immediate ; deferred deferred

That is, the target and prerequisite sections are expanded immediately, and the commands used to construct the target are always deferred. This general rule is true for explicit rules, pattern rules, suffix rules, static pattern rules, and simple prerequisite definitions.

4. Writing Rules

A rule appears in the makefile and says when and how to remake certain files, called the rule's targets (most often only one per rule). It lists the other files that are the prerequisites of the target, and commands to use to create or update the target.

The order of rules is not significant, except for determining the default goal: the target for make to consider, if you do not otherwise specify one. The default goal is the target of the first rule in the first makefile. If the first rule has multiple targets, only the first target is taken as the default. There are two exceptions: a target starting with a period is not a default unless it contains one or more slashes, `/', as well; and, a target that defines a pattern rule has no effect on the default goal. (See section Defining and Redefining Pattern Rules.)

Therefore, we usually write the makefile so that the first rule is the one for compiling the entire program or all the programs described by the makefile (often with a target called `all'). See section Arguments to Specify the Goals.

4.1 Rule Example		An example explained.
4.2 Rule Syntax		General syntax explained.
4.3 Types of Prerequisites		There are two types of prerequisites.
4.4 Using Wildcard Characters in File Names		Using wildcard characters such as `*'.
4.5 Searching Directories for Prerequisites		Searching other directories for source files.
4.6 Phony Targets		Using a target that is not a real file's name.
4.7 Rules without Commands or Prerequisites		You can use a target without commands or prerequisites to mark other targets as phony.
4.8 Empty Target Files to Record Events		When only the date matters and the files are empty.
4.9 Special Built-in Target Names		Targets with special built-in meanings.
4.10 Multiple Targets in a Rule		When to make use of several targets in a rule.
4.11 Multiple Rules for One Target		How to use several rules with the same target.
4.12 Static Pattern Rules		Static pattern rules apply to multiple targets and can vary the prerequisites according to the target name.
4.13 Double-Colon Rules		How to use a special kind of rule to allow several independent rules for one target.
4.14 Generating Prerequisites Automatically		How to automatically generate rules giving prerequisites from source files themselves.

4.1 Rule Example

Here is an example of a rule:

foo.o : foo.c defs.h # module for twiddling the frobs cc -c -g foo.c

Its target is `foo.o' and its prerequisites are `foo.c' and `defs.h'. It has one command, which is `cc -c -g foo.c'. The command line starts with a tab to identify it as a command.

This rule says two things:

• How to decide whether `foo.o' is out of date: it is out of date if it does not exist, or if either `foo.c' or `defs.h' is more recent than it.

• How to update the file `foo.o': by running cc as stated. The command does not explicitly mention `defs.h', but we presume that `foo.c' includes it, and that that is why `defs.h' was added to the prerequisites.

4.2 Rule Syntax

In general, a rule looks like this:

targets : prerequisites command ...

or like this:

targets : prerequisites ; command command ...

The targets are file names, separated by spaces. Wildcard characters may be used (see section Using Wildcard Characters in File Names) and a name of the form `a(m)' represents member m in archive file a (see section Archive Members as Targets). Usually there is only one target per rule, but occasionally there is a reason to have more (see section Multiple Targets in a Rule).

The command lines start with a tab character. The first command may appear on the line after the prerequisites, with a tab character, or may appear on the same line, with a semicolon. Either way, the effect is the same. See section Writing the Commands in Rules.

Because dollar signs are used to start variable references, if you really want a dollar sign in a rule you must write two of them, `$$' (see section How to Use Variables). You may split a long line by inserting a backslash followed by a newline, but this is not required, as make places no limit on the length of a line in a makefile.

A rule tells make two things: when the targets are out of date, and how to update them when necessary.

The criterion for being out of date is specified in terms of the prerequisites, which consist of file names separated by spaces. (Wildcards and archive members (see section 11. Using make to Update Archive Files) are allowed here too.) A target is out of date if it does not exist or if it is older than any of the prerequisites (by comparison of last-modification times). The idea is that the contents of the target file are computed based on information in the prerequisites, so if any of the prerequisites changes, the contents of the existing target file are no longer necessarily valid.

How to update is specified by commands. These are lines to be executed by the shell (normally `sh'), but with some extra features (see section Writing the Commands in Rules).

4.3 Types of Prerequisites

There are actually two different types of prerequisites understood by GNU make: normal prerequisites such as described in the previous section, and order-only prerequisites. A normal prerequisite actually makes two statements: first, it imposes an order of execution of build commands: any commands necessary to build any of a target's prerequisites will be fully executed before any commands necessary to build the target. Second, it imposes a dependency relationship: if any prerequisite is newer than the target, then the target is considered out-of-date and must be rebuilt.

Normally, this is exactly what you want: if a target's prerequisite is updated, then the target should also be updated.

Occasionally, however, you have a situation where you want to impose a specific ordering on the rules to be invoked without forcing the target to be updated if one of those rules is executed. In that case, you want to define order-only prerequisites. Order-only prerequisites can be specified by placing a pipe symbol (|) in the prerequisites list: any prerequisites to the left of the pipe symbol are normal; any prerequisites to the right are order-only:

targets : normal-prerequisites | order-only-prerequisites

The normal prerequisites section may of course be empty. Also, you may still declare multiple lines of prerequisites for the same target: they are appended appropriately. Note that if you declare the same file to be both a normal and an order-only prerequisite, the normal prerequisite takes precedence (since they are a strict superset of the behavior of an order-only prerequisite).

4.4 Using Wildcard Characters in File Names

A single file name can specify many files using wildcard characters. The wildcard characters in make are `*', `?' and `[...]', the same as in the Bourne shell. For example, `*.c' specifies a list of all the files (in the working directory) whose names end in `.c'.

The character `~' at the beginning of a file name also has special significance. If alone, or followed by a slash, it represents your home directory. For example `~/bin' expands to `/home/you/bin'. If the `~' is followed by a word, the string represents the home directory of the user named by that word. For example `~john/bin' expands to `/home/john/bin'. On systems which don't have a home directory for each user (such as MS-DOS or MS-Windows), this functionality can be simulated by setting the environment variable HOME.

Wildcard expansion happens automatically in targets, in prerequisites, and in commands (where the shell does the expansion). In other contexts, wildcard expansion happens only if you request it explicitly with the wildcard function.

The special significance of a wildcard character can be turned off by preceding it with a backslash. Thus, `foo\*bar' would refer to a specific file whose name consists of `foo', an asterisk, and `bar'.

4.4.1 Wildcard Examples		Several examples
4.4.2 Pitfalls of Using Wildcards		Problems to avoid.
4.4.3 The Function wildcard		How to cause wildcard expansion where it does not normally take place.

4.4.1 Wildcard Examples

Wildcards can be used in the commands of a rule, where they are expanded by the shell. For example, here is a rule to delete all the object files:

clean: rm -f *.o

Wildcards are also useful in the prerequisites of a rule. With the following rule in the makefile, `make print' will print all the `.c' files that have changed since the last time you printed them:

print: *.c lpr -p $? touch print

This rule uses `print' as an empty target file; see Empty Target Files to Record Events. (The automatic variable `$?' is used to print only those files that have changed; see Automatic Variables.)

Wildcard expansion does not happen when you define a variable. Thus, if you write this:

objects = *.o

then the value of the variable objects is the actual string `*.o'. However, if you use the value of objects in a target, prerequisite or command, wildcard expansion will take place at that time. To set objects to the expansion, instead use:

objects := $(wildcard *.o)

See section 4.4.3 The Function wildcard.

4.4.2 Pitfalls of Using Wildcards

Now here is an example of a naive way of using wildcard expansion, that does not do what you would intend. Suppose you would like to say that the executable file `foo' is made from all the object files in the directory, and you write this:

objects = *.o foo : $(objects) cc -o foo $(CFLAGS) $(objects)

The value of objects is the actual string `*.o'. Wildcard expansion happens in the rule for `foo', so that each existing `.o' file becomes a prerequisite of `foo' and will be recompiled if necessary.

But what if you delete all the `.o' files? When a wildcard matches no files, it is left as it is, so then `foo' will depend on the oddly-named file `*.o'. Since no such file is likely to exist, make will give you an error saying it cannot figure out how to make `*.o'. This is not what you want!

Actually it is possible to obtain the desired result with wildcard expansion, but you need more sophisticated techniques, including the wildcard function and string substitution. See section The Function wildcard.

Microsoft operating systems (MS-DOS and MS-Windows) use backslashes to separate directories in pathnames, like so:

c:\foo\bar\baz.c

This is equivalent to the Unix-style `c:/foo/bar/baz.c' (the `c:' part is the so-called drive letter). When make runs on these systems, it supports backslashes as well as the Unix-style forward slashes in pathnames. However, this support does not include the wildcard expansion, where backslash is a quote character. Therefore, you must use Unix-style slashes in these cases.

4.4.3 The Function wildcard

Wildcard expansion happens automatically in rules. But wildcard expansion does not normally take place when a variable is set, or inside the arguments of a function. If you want to do wildcard expansion in such places, you need to use the wildcard function, like this:

$(wildcard pattern...)

This string, used anywhere in a makefile, is replaced by a space-separated list of names of existing files that match one of the given file name patterns. If no existing file name matches a pattern, then that pattern is omitted from the output of the wildcard function. Note that this is different from how unmatched wildcards behave in rules, where they are used verbatim rather than ignored (see section 4.4.2 Pitfalls of Using Wildcards).

One use of the wildcard function is to get a list of all the C source files in a directory, like this:

$(wildcard *.c)

We can change the list of C source files into a list of object files by replacing the `.c' suffix with `.o' in the result, like this:

$(patsubst %.c,%.o,$(wildcard *.c))

(Here we have used another function, patsubst. See section Functions for String Substitution and Analysis.)

Thus, a makefile to compile all C source files in the directory and then link them together could be written as follows:

objects := $(patsubst %.c,%.o,$(wildcard *.c)) foo : $(objects) cc -o foo $(objects)

(This takes advantage of the implicit rule for compiling C programs, so there is no need to write explicit rules for compiling the files. See section The Two Flavors of Variables, for an explanation of `:=', which is a variant of `='.)

4.5 Searching Directories for Prerequisites

For large systems, it is often desirable to put sources in a separate directory from the binaries. The directory search features of make facilitate this by searching several directories automatically to find a prerequisite. When you redistribute the files among directories, you do not need to change the individual rules, just the search paths.

4.5.1 VPATH: Search Path for All Prerequisites		Specifying a search path that applies to every prerequisite.
4.5.2 The vpath Directive		Specifying a search path for a specified class of names.
4.5.3 How Directory Searches are Performed		When and how search paths are applied.
4.5.4 Writing Shell Commands with Directory Search		How to write shell commands that work together with search paths.
4.5.5 Directory Search and Implicit Rules		How search paths affect implicit rules.
4.5.6 Directory Search for Link Libraries		Directory search for link libraries.

4.5.1 VPATH: Search Path for All Prerequisites

The value of the make variable VPATH specifies a list of directories that make should search. Most often, the directories are expected to contain prerequisite files that are not in the current directory; however, VPATH specifies a search list that make applies for all files, including files which are targets of rules.

Thus, if a file that is listed as a target or prerequisite does not exist in the current directory, make searches the directories listed in VPATH for a file with that name. If a file is found in one of them, that file may become the prerequisite (see below). Rules may then specify the names of files in the prerequisite list as if they all existed in the current directory. See section Writing Shell Commands with Directory Search.

In the VPATH variable, directory names are separated by colons or blanks. The order in which directories are listed is the order followed by make in its search. (On MS-DOS and MS-Windows, semi-colons are used as separators of directory names in VPATH, since the colon can be used in the pathname itself, after the drive letter.)

For example,

VPATH = src:../headers

specifies a path containing two directories, `src' and `../headers', which make searches in that order.

With this value of VPATH, the following rule,

foo.o : foo.c

is interpreted as if it were written like this:

foo.o : src/foo.c

assuming the file `foo.c' does not exist in the current directory but is found in the directory `src'.

4.5.2 The vpath Directive

Similar to the VPATH variable, but more selective, is the vpath directive (note lower case), which allows you to specify a search path for a particular class of file names: those that match a particular pattern. Thus you can supply certain search directories for one class of file names and other directories (or none) for other file names.

There are three forms of the vpath directive:

vpath pattern directories

Specify the search path directories for file names that match pattern.

The search path, directories, is a list of directories to be searched, separated by colons (semi-colons on MS-DOS and MS-Windows) or blanks, just like the search path used in the VPATH variable.

vpath pattern

Clear out the search path associated with pattern.

vpath

Clear all search paths previously specified with vpath directives.

A vpath pattern is a string containing a `%' character. The string must match the file name of a prerequisite that is being searched for, the `%' character matching any sequence of zero or more characters (as in pattern rules; see section Defining and Redefining Pattern Rules). For example, %.h matches files that end in .h. (If there is no `%', the pattern must match the prerequisite exactly, which is not useful very often.)

`%' characters in a vpath directive's pattern can be quoted with preceding backslashes (`\'). Backslashes that would otherwise quote `%' characters can be quoted with more backslashes. Backslashes that quote `%' characters or other backslashes are removed from the pattern before it is compared to file names. Backslashes that are not in danger of quoting `%' characters go unmolested.

When a prerequisite fails to exist in the current directory, if the pattern in a vpath directive matches the name of the prerequisite file, then the directories in that directive are searched just like (and before) the directories in the VPATH variable.

For example,

vpath %.h ../headers

tells make to look for any prerequisite whose name ends in `.h' in the directory `../headers' if the file is not found in the current directory.

If several vpath patterns match the prerequisite file's name, then make processes each matching vpath directive one by one, searching all the directories mentioned in each directive. make handles multiple vpath directives in the order in which they appear in the makefile; multiple directives with the same pattern are independent of each other.

Thus,

vpath %.c foo vpath % blish vpath %.c bar

will look for a file ending in `.c' in `foo', then `blish', then `bar', while

vpath %.c foo:bar vpath % blish

will look for a file ending in `.c' in `foo', then `bar', then `blish'.

4.5.3 How Directory Searches are Performed

When a prerequisite is found through directory search, regardless of type (general or selective), the pathname located may not be the one that make actually provides you in the prerequisite list. Sometimes the path discovered through directory search is thrown away.

The algorithm make uses to decide whether to keep or abandon a path found via directory search is as follows:

1. If a target file does not exist at the path specified in the makefile, directory search is performed.

2. If the directory search is successful, that path is kept and this file is tentatively stored as the target.

3. All prerequisites of this target are examined using this same method.

4. After processing the prerequisites, the target may or may not need to be rebuilt:
   1. If the target does not need to be rebuilt, the path to the file found during directory search is used for any prerequisite lists which contain this target. In short, if make doesn't need to rebuild the target then you use the path found via directory search.

   2. If the target does need to be rebuilt (is out-of-date), the pathname found during directory search is thrown away, and the target is rebuilt using the file name specified in the makefile. In short, if make must rebuild, then the target is rebuilt locally, not in the directory found via directory search.

This algorithm may seem complex, but in practice it is quite often exactly what you want.

Other versions of make use a simpler algorithm: if the file does not exist, and it is found via directory search, then that pathname is always used whether or not the target needs to be built. Thus, if the target is rebuilt it is created at the pathname discovered during directory search.

If, in fact, this is the behavior you want for some or all of your directories, you can use the GPATH variable to indicate this to make.

GPATH has the same syntax and format as VPATH (that is, a space- or colon-delimited list of pathnames). If an out-of-date target is found by directory search in a directory that also appears in GPATH, then that pathname is not thrown away. The target is rebuilt using the expanded path.

4.5.4 Writing Shell Commands with Directory Search

When a prerequisite is found in another directory through directory search, this cannot change the commands of the rule; they will execute as written. Therefore, you must write the commands with care so that they will look for the prerequisite in the directory where make finds it.

This is done with the automatic variables such as `$^' (see section Automatic Variables). For instance, the value of `$^' is a list of all the prerequisites of the rule, including the names of the directories in which they were found, and the value of `$@' is the target. Thus:

foo.o : foo.c cc -c $(CFLAGS) $^ -o $@

(The variable CFLAGS exists so you can specify flags for C compilation by implicit rules; we use it here for consistency so it will affect all C compilations uniformly; see section Variables Used by Implicit Rules.)

Often the prerequisites include header files as well, which you do not want to mention in the commands. The automatic variable `$<' is just the first prerequisite:

VPATH = src:../headers foo.o : foo.c defs.h hack.h cc -c $(CFLAGS) $< -o $@

4.5.5 Directory Search and Implicit Rules

The search through the directories specified in VPATH or with vpath also happens during consideration of implicit rules (see section Using Implicit Rules).

For example, when a file `foo.o' has no explicit rule, make considers implicit rules, such as the built-in rule to compile `foo.c' if that file exists. If such a file is lacking in the current directory, the appropriate directories are searched for it. If `foo.c' exists (or is mentioned in the makefile) in any of the directories, the implicit rule for C compilation is applied.

The commands of implicit rules normally use automatic variables as a matter of necessity; consequently they will use the file names found by directory search with no extra effort.

4.5.6 Directory Search for Link Libraries

Directory search applies in a special way to libraries used with the linker. This special feature comes into play when you write a prerequisite whose name is of the form `-lname'. (You can tell something strange is going on here because the prerequisite is normally the name of a file, and the file name of a library generally looks like `libname.a', not like `-lname'.)

When a prerequisite's name has the form `-lname', make handles it specially by searching for the file `libname.so' in the current directory, in directories specified by matching vpath search paths and the VPATH search path, and then in the directories `/lib', `/usr/lib', and `prefix/lib' (normally `/usr/local/lib', but MS-DOS/MS-Windows versions of make behave as if prefix is defined to be the root of the DJGPP installation tree).

If that file is not found, then the file `libname.a' is searched for, in the same directories as above.

For example, if there is a `/usr/lib/libcurses.a' library on your system (and no `/usr/lib/libcurses.so' file), then

foo : foo.c -lcurses cc $^ -o $@

would cause the command `cc foo.c /usr/lib/libcurses.a -o foo' to be executed when `foo' is older than `foo.c' or than `/usr/lib/libcurses.a'.

Although the default set of files to be searched for is `libname.so' and `libname.a', this is customizable via the .LIBPATTERNS variable. Each word in the value of this variable is a pattern string. When a prerequisite like `-lname' is seen, make will replace the percent in each pattern in the list with name and perform the above directory searches using that library filename. If no library is found, the next word in the list will be used.

The default value for .LIBPATTERNS is "`lib%.so lib%.a'", which provides the default behavior described above.

You can turn off link library expansion completely by setting this variable to an empty value.

4.6 Phony Targets

A phony target is one that is not really the name of a file. It is just a name for some commands to be executed when you make an explicit request. There are two reasons to use a phony target: to avoid a conflict with a file of the same name, and to improve performance.

If you write a rule whose commands will not create the target file, the commands will be executed every time the target comes up for remaking. Here is an example:

clean: rm *.o temp

Because the rm command does not create a file named `clean', probably no such file will ever exist. Therefore, the rm command will be executed every time you say `make clean'.

The phony target will cease to work if anything ever does create a file named `clean' in this directory. Since it has no prerequisites, the file `clean' would inevitably be considered up to date, and its commands would not be executed. To avoid this problem, you can explicitly declare the target to be phony, using the special target .PHONY (see section Special Built-in Target Names) as follows:

.PHONY : clean

Once this is done, `make clean' will run the commands regardless of whether there is a file named `clean'.

Since it knows that phony targets do not name actual files that could be remade from other files, make skips the implicit rule search for phony targets (see section 10. Using Implicit Rules). This is why declaring a target phony is good for performance, even if you are not worried about the actual file existing.

Thus, you first write the line that states that clean is a phony target, then you write the rule, like this:

.PHONY: clean clean: rm *.o temp

Another example of the usefulness of phony targets is in conjunction with recursive invocations of make (for more information, see Recursive Use of make). In this case the makefile will often contain a variable which lists a number of subdirectories to be built. One way to handle this is with one rule whose command is a shell loop over the subdirectories, like this:

SUBDIRS = foo bar baz subdirs: for dir in $(SUBDIRS); do \ $(MAKE) -C $$dir; \ done

There are a few problems with this method, however. First, any error detected in a submake is not noted by this rule, so it will continue to build the rest of the directories even when one fails. This can be overcome by adding shell commands to note the error and exit, but then it will do so even if make is invoked with the -k option, which is unfortunate. Second, and perhaps more importantly, you cannot take advantage of the parallel build capabilities of make using this method, since there is only one rule.

By declaring the subdirectories as phony targets (you must do this as the subdirectory obviously always exists; otherwise it won't be built) you can remove these problems:

SUBDIRS = foo bar baz .PHONY: subdirs $(SUBDIRS) subdirs: $(SUBDIRS) $(SUBDIRS): $(MAKE) -C $@ foo: baz

Here we've also declared that the `foo' subdirectory cannot be built until after the `baz' subdirectory is complete; this kind of relationship declaration is particularly important when attempting parallel builds.

A phony target should not be a prerequisite of a real target file; if it is, its commands are run every time make goes to update that file. As long as a phony target is never a prerequisite of a real target, the phony target commands will be executed only when the phony target is a specified goal (see section Arguments to Specify the Goals).

Phony targets can have prerequisites. When one directory contains multiple programs, it is most convenient to describe all of the programs in one makefile `./Makefile'. Since the target remade by default will be the first one in the makefile, it is common to make this a phony target named `all' and give it, as prerequisites, all the individual programs. For example:

all : prog1 prog2 prog3 .PHONY : all prog1 : prog1.o utils.o cc -o prog1 prog1.o utils.o prog2 : prog2.o cc -o prog2 prog2.o prog3 : prog3.o sort.o utils.o cc -o prog3 prog3.o sort.o utils.o

Now you can say just `make' to remake all three programs, or specify as arguments the ones to remake (as in `make prog1 prog3').

When one phony target is a prerequisite of another, it serves as a subroutine of the other. For example, here `make cleanall' will delete the object files, the difference files, and the file `program':

.PHONY: cleanall cleanobj cleandiff cleanall : cleanobj cleandiff rm program cleanobj : rm *.o cleandiff : rm *.diff

4.7 Rules without Commands or Prerequisites

If a rule has no prerequisites or commands, and the target of the rule is a nonexistent file, then make imagines this target to have been updated whenever its rule is run. This implies that all targets depending on this one will always have their commands run.

An example will illustrate this:

clean: FORCE rm $(objects) FORCE:

Here the target `FORCE' satisfies the special conditions, so the target `clean' that depends on it is forced to run its commands. There is nothing special about the name `FORCE', but that is one name commonly used this way.

As you can see, using `FORCE' this way has the same results as using `.PHONY: clean'.

Using `.PHONY' is more explicit and more efficient. However, other versions of make do not support `.PHONY'; thus `FORCE' appears in many makefiles. See section 4.6 Phony Targets.

4.8 Empty Target Files to Record Events

The empty target is a variant of the phony target; it is used to hold commands for an action that you request explicitly from time to time. Unlike a phony target, this target file can really exist; but the file's contents do not matter, and usually are empty.

The purpose of the empty target file is to record, with its last-modification time, when the rule's commands were last executed. It does so because one of the commands is a touch command to update the target file.

The empty target file should have some prerequisites (otherwise it doesn't make sense). When you ask to remake the empty target, the commands are executed if any prerequisite is more recent than the target; in other words, if a prerequisite has changed since the last time you remade the target. Here is an example:

print: foo.c bar.c lpr -p $? touch print

With this rule, `make print' will execute the lpr command if either source file has changed since the last `make print'. The automatic variable `$?' is used to print only those files that have changed (see section Automatic Variables).

4.9 Special Built-in Target Names

Certain names have special meanings if they appear as targets.

.PHONY

The prerequisites of the special target .PHONY are considered to be phony targets. When it is time to consider such a target, make will run its commands unconditionally, regardless of whether a file with that name exists or what its last-modification time is. See section Phony Targets.

.SUFFIXES

The prerequisites of the special target .SUFFIXES are the list of suffixes to be used in checking for suffix rules. See section Old-Fashioned Suffix Rules.

.DEFAULT

The commands specified for .DEFAULT are used for any target for which no rules are found (either explicit rules or implicit rules). See section 10.6 Defining Last-Resort Default Rules. If .DEFAULT commands are specified, every file mentioned as a prerequisite, but not as a target in a rule, will have these commands executed on its behalf. See section Implicit Rule Search Algorithm.

.PRECIOUS

The targets which .PRECIOUS depends on are given the following special treatment: if make is killed or interrupted during the execution of their commands, the target is not deleted. See section Interrupting or Killing make. Also, if the target is an intermediate file, it will not be deleted after it is no longer needed, as is normally done. See section Chains of Implicit Rules. In this latter respect it overlaps with the .SECONDARY special target.

You can also list the target pattern of an implicit rule (such as `%.o') as a prerequisite file of the special target .PRECIOUS to preserve intermediate files created by rules whose target patterns match that file's name.

.INTERMEDIATE

The targets which .INTERMEDIATE depends on are treated as intermediate files. See section Chains of Implicit Rules. .INTERMEDIATE with no prerequisites has no effect.

.SECONDARY

The targets which .SECONDARY depends on are treated as intermediate files, except that they are never automatically deleted. See section Chains of Implicit Rules.

.SECONDARY with no prerequisites causes all targets to be treated as secondary (i.e., no target is removed because it is considered intermediate).

.DELETE_ON_ERROR

If .DELETE_ON_ERROR is mentioned as a target anywhere in the makefile, then make will delete the target of a rule if it has changed and its commands exit with a nonzero exit status, just as it does when it receives a signal. See section Errors in Commands.

.IGNORE

If you specify prerequisites for .IGNORE, then make will ignore errors in execution of the commands run for those particular files. The commands for .IGNORE are not meaningful.

If mentioned as a target with no prerequisites, .IGNORE says to ignore errors in execution of commands for all files. This usage of `.IGNORE' is supported only for historical compatibility. Since this affects every command in the makefile, it is not very useful; we recommend you use the more selective ways to ignore errors in specific commands. See section Errors in Commands.

.LOW_RESOLUTION_TIME

If you specify prerequisites for .LOW_RESOLUTION_TIME, make assumes that these files are created by commands that generate low resolution time stamps. The commands for .LOW_RESOLUTION_TIME are not meaningful.

The high resolution file time stamps of many modern hosts lessen the chance of make incorrectly concluding that a file is up to date. Unfortunately, these hosts provide no way to set a high resolution file time stamp, so commands like `cp -p' that explicitly set a file's time stamp must discard its subsecond part. If a file is created by such a command, you should list it as a prerequisite of .LOW_RESOLUTION_TIME so that make does not mistakenly conclude that the file is out of date. For example:

.LOW_RESOLUTION_TIME: dst dst: src cp -p src dst

Since `cp -p' discards the subsecond part of `src''s time stamp, `dst' is typically slightly older than `src' even when it is up to date. The .LOW_RESOLUTION_TIME line causes make to consider `dst' to be up to date if its time stamp is at the start of the same second that `src''s time stamp is in.

Due to a limitation of the archive format, archive member time stamps are always low resolution. You need not list archive members as prerequisites of .LOW_RESOLUTION_TIME, as make does this automatically.

.SILENT

If you specify prerequisites for .SILENT, then make will not print the commands to remake those particular files before executing them. The commands for .SILENT are not meaningful.

If mentioned as a target with no prerequisites, .SILENT says not to print any commands before executing them. This usage of `.SILENT' is supported only for historical compatibility. We recommend you use the more selective ways to silence specific commands. See section Command Echoing. If you want to silence all commands for a particular run of make, use the `-s' or `--silent' option (see section 9.7 Summary of Options).

.EXPORT_ALL_VARIABLES

Simply by being mentioned as a target, this tells make to export all variables to child processes by default. See section Communicating Variables to a Sub-make.

.NOTPARALLEL

If .NOTPARALLEL is mentioned as a target, then this invocation of make will be run serially, even if the `-j' option is given. Any recursively invoked make command will still be run in parallel (unless its makefile contains this target). Any prerequisites on this target are ignored.

Any defined implicit rule suffix also counts as a special target if it appears as a target, and so does the concatenation of two suffixes, such as `.c.o'. These targets are suffix rules, an obsolete way of defining implicit rules (but a way still widely used). In principle, any target name could be special in this way if you break it in two and add both pieces to the suffix list. In practice, suffixes normally begin with `.', so these special target names also begin with `.'. See section Old-Fashioned Suffix Rules.

4.10 Multiple Targets in a Rule

A rule with multiple targets is equivalent to writing many rules, each with one target, and all identical aside from that. The same commands apply to all the targets, but their effects may vary because you can substitute the actual target name into the command using `$@'. The rule contributes the same prerequisites to all the targets also.

This is useful in two cases.

• You want just prerequisites, no commands. For example:

  kbd.o command.o files.o: command.h

  gives an additional prerequisite to each of the three object files mentioned.

• Similar commands work for all the targets. The commands do not need to be absolutely identical, since the automatic variable `$@' can be used to substitute the particular target to be remade into the commands (see section Automatic Variables). For example:

  bigoutput littleoutput : text.g generate text.g -$(subst output,,$@) > $@

  is equivalent to

  bigoutput : text.g generate text.g -big > bigoutput littleoutput : text.g generate text.g -little > littleoutput

  Here we assume the hypothetical program generate makes two types of output, one if given `-big' and one if given `-little'. See section Functions for String Substitution and Analysis, for an explanation of the subst function.

Suppose you would like to vary the prerequisites according to the target, much as the variable `$@' allows you to vary the commands. You cannot do this with multiple targets in an ordinary rule, but you can do it with a static pattern rule. See section Static Pattern Rules.

4.11 Multiple Rules for One Target

One file can be the target of several rules. All the prerequisites mentioned in all the rules are merged into one list of prerequisites for the target. If the target is older than any prerequisite from any rule, the commands are executed.

There can only be one set of commands to be executed for a file. If more than one rule gives commands for the same file, make uses the last set given and prints an error message. (As a special case, if the file's name begins with a dot, no error message is printed. This odd behavior is only for compatibility with other implementations of make... you should avoid using it). Occasionally it is useful to have the same target invoke multiple commands which are defined in different parts of your makefile; you can use double-colon rules (see section 4.13 Double-Colon Rules) for this.

An extra rule with just prerequisites can be used to give a few extra prerequisites to many files at once. For example, makefiles often have a variable, such as objects, containing a list of all the compiler output files in the system being made. An easy way to say that all of them must be recompiled if `config.h' changes is to write the following:

objects = foo.o bar.o foo.o : defs.h bar.o : defs.h test.h $(objects) : config.h

This could be inserted or taken out without changing the rules that really specify how to make the object files, making it a convenient form to use if you wish to add the additional prerequisite intermittently.

Another wrinkle is that the additional prerequisites could be specified with a variable that you set with a command argument to make (see section Overriding Variables). For example,

extradeps= $(objects) : $(extradeps)

means that the command `make extradeps=foo.h' will consider `foo.h' as a prerequisite of each object file, but plain `make' will not.

If none of the explicit rules for a target has commands, then make searches for an applicable implicit rule to find some commands see section Using Implicit Rules).

4.12 Static Pattern Rules

Static pattern rules are rules which specify multiple targets and construct the prerequisite names for each target based on the target name. They are more general than ordinary rules with multiple targets because the targets do not have to have identical prerequisites. Their prerequisites must be analogous, but not necessarily identical.

4.12.1 Syntax of Static Pattern Rules		The syntax of static pattern rules.
4.12.2 Static Pattern Rules versus Implicit Rules		When are they better than implicit rules?

4.12.1 Syntax of Static Pattern Rules

Here is the syntax of a static pattern rule:

targets ...: target-pattern: prereq-patterns ... commands ...

The targets list specifies the targets that the rule applies to. The targets can contain wildcard characters, just like the targets of ordinary rules (see section Using Wildcard Characters in File Names).

The target-pattern and prereq-patterns say how to compute the prerequisites of each target. Each target is matched against the target-pattern to extract a part of the target name, called the stem. This stem is substituted into each of the prereq-patterns to make the prerequisite names (one from each prereq-pattern).

Each pattern normally contains the character `%' just once. When the target-pattern matches a target, the `%' can match any part of the target name; this part is called the stem. The rest of the pattern must match exactly. For example, the target `foo.o' matches the pattern `%.o', with `foo' as the stem. The targets `foo.c' and `foo.out' do not match that pattern.

The prerequisite names for each target are made by substituting the stem for the `%' in each prerequisite pattern. For example, if one prerequisite pattern is `%.c', then substitution of the stem `foo' gives the prerequisite name `foo.c'. It is legitimate to write a prerequisite pattern that does not contain `%'; then this prerequisite is the same for all targets.

`%' characters in pattern rules can be quoted with preceding backslashes (`\'). Backslashes that would otherwise quote `%' characters can be quoted with more backslashes. Backslashes that quote `%' characters or other backslashes are removed from the pattern before it is compared to file names or has a stem substituted into it. Backslashes that are not in danger of quoting `%' characters go unmolested. For example, the pattern `the\%weird\\%pattern\\' has `the%weird\' preceding the operative `%' character, and `pattern\\' following it. The final two backslashes are left alone because they cannot affect any `%' character.

Here is an example, which compiles each of `foo.o' and `bar.o' from the corresponding `.c' file:

objects = foo.o bar.o all: $(objects) $(objects): %.o: %.c $(CC) -c $(CFLAGS) $< -o $@

Here `$<' is the automatic variable that holds the name of the prerequisite and `$@' is the automatic variable that holds the name of the target; see Automatic Variables.

Each target specified must match the target pattern; a warning is issued for each target that does not. If you have a list of files, only some of which will match the pattern, you can use the filter function to remove nonmatching file names (see section Functions for String Substitution and Analysis):

files = foo.elc bar.o lose.o $(filter %.o,$(files)): %.o: %.c $(CC) -c $(CFLAGS) $< -o $@ $(filter %.elc,$(files)): %.elc: %.el emacs -f batch-byte-compile $<

In this example the result of `$(filter %.o,$(files))' is `bar.o lose.o', and the first static pattern rule causes each of these object files to be updated by compiling the corresponding C source file. The result of `$(filter %.elc,$(files))' is `foo.elc', so that file is made from `foo.el'.

Another example shows how to use $* in static pattern rules:

bigoutput littleoutput : %output : text.g generate text.g -$* > $@

When the generate command is run, $* will expand to the stem, either `big' or `little'.

4.12.2 Static Pattern Rules versus Implicit Rules

A static pattern rule has much in common with an implicit rule defined as a pattern rule (see section Defining and Redefining Pattern Rules). Both have a pattern for the target and patterns for constructing the names of prerequisites. The difference is in how make decides when the rule applies.

An implicit rule can apply to any target that matches its pattern, but it does apply only when the target has no commands otherwise specified, and only when the prerequisites can be found. If more than one implicit rule appears applicable, only one applies; the choice depends on the order of rules.

By contrast, a static pattern rule applies to the precise list of targets that you specify in the rule. It cannot apply to any other target and it invariably does apply to each of the targets specified. If two conflicting rules apply, and both have commands, that's an error.

The static pattern rule can be better than an implicit rule for these reasons:

• You may wish to override the usual implicit rule for a few files whose names cannot be categorized syntactically but can be given in an explicit list.

• If you cannot be sure of the precise contents of the directories you are using, you may not be sure which other irrelevant files might lead make to use the wrong implicit rule. The choice might depend on the order in which the implicit rule search is done. With static pattern rules, there is no uncertainty: each rule applies to precisely the targets specified.

4.13 Double-Colon Rules

Double-colon rules are rules written with `::' instead of `:' after the target names. They are handled differently from ordinary rules when the same target appears in more than one rule.

When a target appears in multiple rules, all the rules must be the same type: all ordinary, or all double-colon. If they are double-colon, each of them is independent of the others. Each double-colon rule's commands are executed if the target is older than any prerequisites of that rule. If there are no prerequisites for that rule, its commands are always executed (even if the target already exists). This can result in executing none, any, or all of the double-colon rules.

Double-colon rules with the same target are in fact completely separate from one another. Each double-colon rule is processed individually, just as rules with different targets are processed.

The double-colon rules for a target are executed in the order they appear in the makefile. However, the cases where double-colon rules really make sense are those where the order of executing the commands would not matter.

Double-colon rules are somewhat obscure and not often very useful; they provide a mechanism for cases in which the method used to update a target differs depending on which prerequisite files caused the update, and such cases are rare.

Each double-colon rule should specify commands; if it does not, an implicit rule will be used if one applies. See section Using Implicit Rules.

4.14 Generating Prerequisites Automatically

In the makefile for a program, many of the rules you need to write often say only that some object file depends on some header file. For example, if `main.c' uses `defs.h' via an #include, you would write:

main.o: defs.h

You need this rule so that make knows that it must remake `main.o' whenever `defs.h' changes. You can see that for a large program you would have to write dozens of such rules in your makefile. And, you must always be very careful to update the makefile every time you add or remove an #include.

To avoid this hassle, most modern C compilers can write these rules for you, by looking at the #include lines in the source files. Usually this is done with the `-M' option to the compiler. For example, the command:

cc -M main.c

generates the output:

main.o : main.c defs.h

Thus you no longer have to write all those rules yourself. The compiler will do it for you.

Note that such a prerequisite constitutes mentioning `main.o' in a makefile, so it can never be considered an intermediate file by implicit rule search. This means that make won't ever remove the file after using it; see section Chains of Implicit Rules.

With old make programs, it was traditional practice to use this compiler feature to generate prerequisites on demand with a command like `make depend'. That command would create a file `depend' containing all the automatically-generated prerequisites; then the makefile could use include to read them in (see section 3.3 Including Other Makefiles).

In GNU make, the feature of remaking makefiles makes this practice obsolete--you need never tell make explicitly to regenerate the prerequisites, because it always regenerates any makefile that is out of date. See section 3.7 How Makefiles Are Remade.

The practice we recommend for automatic prerequisite generation is to have one makefile corresponding to each source file. For each source file `name.c' there is a makefile `name.d' which lists what files the object file `name.o' depends on. That way only the source files that have changed need to be rescanned to produce the new prerequisites.

Here is the pattern rule to generate a file of prerequisites (i.e., a makefile) called `name.d' from a C source file called `name.c':

%.d: %.c @set -e; rm -f $@; \ $(CC) -M $(CPPFLAGS) $< > $@.$$$$; \ sed 's,\($*\)\.o[ :]*,\1.o $@ : ,g' < $@.$$$$ > $@; \ rm -f $@.$$$$

See section 10.5 Defining and Redefining Pattern Rules, for information on defining pattern rules. The `-e' flag to the shell causes it to exit immediately if the $(CC) command (or any other command) fails (exits with a nonzero status).

With the GNU C compiler, you may wish to use the `-MM' flag instead of `-M'. This omits prerequisites on system header files. See section `Options Controlling the Preprocessor' in Using GNU CC, for details.

The purpose of the sed command is to translate (for example):

main.o : main.c defs.h

into:

main.o main.d : main.c defs.h

This makes each `.d' file depend on all the source and header files that the corresponding `.o' file depends on. make then knows it must regenerate the prerequisites whenever any of the source or header files changes.

Once you've defined the rule to remake the `.d' files, you then use the include directive to read them all in. See section 3.3 Including Other Makefiles. For example:

sources = foo.c bar.c include $(sources:.c=.d)

(This example uses a substitution variable reference to translate the list of source files `foo.c bar.c' into a list of prerequisite makefiles, `foo.d bar.d'. See section 6.3.1 Substitution References, for full information on substitution references.) Since the `.d' files are makefiles like any others, make will remake them as necessary with no further work from you. See section 3.7 How Makefiles Are Remade.

Note that the `.d' files contain target definitions; you should be sure to place the include directive after the first, default target in your makefiles or run the risk of having a random object file become the default target. See section 2.3 How make Processes a Makefile.

5. Writing the Commands in Rules

The commands of a rule consist of shell command lines to be executed one by one. Each command line must start with a tab, except that the first command line may be attached to the target-and-prerequisites line with a semicolon in between. Blank lines and lines of just comments may appear among the command lines; they are ignored. (But beware, an apparently "blank" line that begins with a tab is not blank! It is an empty command; see section 5.8 Using Empty Commands.)

Users use many different shell programs, but commands in makefiles are always interpreted by `/bin/sh' unless the makefile specifies otherwise. See section Command Execution.

The shell that is in use determines whether comments can be written on command lines, and what syntax they use. When the shell is `/bin/sh', a `#' starts a comment that extends to the end of the line. The `#' does not have to be at the beginning of a line. Text on a line before a `#' is not part of the comment.

5.1 Command Echoing		How to control when commands are echoed.
5.2 Command Execution		How commands are executed.
5.3 Parallel Execution		How commands can be executed in parallel.
5.4 Errors in Commands		What happens after a command execution error.
5.5 Interrupting or Killing make		What happens when a command is interrupted.
5.6 Recursive Use of make		Invoking make from makefiles.
5.7 Defining Canned Command Sequences		Defining canned sequences of commands.
5.8 Using Empty Commands		Defining useful, do-nothing commands.

5.1 Command Echoing

Normally make prints each command line before it is executed. We call this echoing because it gives the appearance that you are typing the commands yourself.

When a line starts with `@', the echoing of that line is suppressed. The `@' is discarded before the command is passed to the shell. Typically you would use this for a command whose only effect is to print something, such as an echo command to indicate progress through the makefile:

@echo About to make distribution files

When make is given the flag `-n' or `--just-print' it only echoes commands, it won't execute them. See section Summary of Options. In this case and only this case, even the commands starting with `@' are printed. This flag is useful for finding out which commands make thinks are necessary without actually doing them.

The `-s' or `--silent' flag to make prevents all echoing, as if all commands started with `@'. A rule in the makefile for the special target .SILENT without prerequisites has the same effect (see section Special Built-in Target Names). .SILENT is essentially obsolete since `@' is more flexible.

5.2 Command Execution

When it is time to execute commands to update a target, they are executed by making a new subshell for each line. (In practice, make may take shortcuts that do not affect the results.)

Please note: this implies that shell commands such as cd that set variables local to each process will not affect the following command lines. (2) If you want to use cd to affect the next command, put the two on a single line with a semicolon between them. Then make will consider them a single command and pass them, together, to a shell which will execute them in sequence. For example:

foo : bar/lose cd bar; gobble lose > ../foo

If you would like to split a single shell command into multiple lines of text, you must use a backslash at the end of all but the last subline. Such a sequence of lines is combined into a single line, by deleting the backslash-newline sequences, before passing it to the shell. Thus, the following is equivalent to the preceding example:

foo : bar/lose cd bar; \ gobble lose > ../foo

The program used as the shell is taken from the variable SHELL. By default, the program `/bin/sh' is used.

On MS-DOS, if SHELL is not set, the value of the variable COMSPEC (which is always set) is used instead.

The processing of lines that set the variable SHELL in Makefiles is different on MS-DOS. The stock shell, `command.com', is ridiculously limited in its functionality and many users of make tend to install a replacement shell. Therefore, on MS-DOS, make examines the value of SHELL, and changes its behavior based on whether it points to a Unix-style or DOS-style shell. This allows reasonable functionality even if SHELL points to `command.com'.

If SHELL points to a Unix-style shell, make on MS-DOS additionally checks whether that shell can indeed be found; if not, it ignores the line that sets SHELL. In MS-DOS, GNU make searches for the shell in the following places:

1. In the precise place pointed to by the value of SHELL. For example, if the makefile specifies `SHELL = /bin/sh', make will look in the directory `/bin' on the current drive.

2. In the current directory.

3. In each of the directories in the PATH variable, in order.

In every directory it examines, make will first look for the specific file (`sh' in the example above). If this is not found, it will also look in that directory for that file with one of the known extensions which identify executable files. For example `.exe', `.com', `.bat', `.btm', `.sh', and some others.

If any of these attempts is successful, the value of SHELL will be set to the full pathname of the shell as found. However, if none of these is found, the value of SHELL will not be changed, and thus the line that sets it will be effectively ignored. This is so make will only support features specific to a Unix-style shell if such a shell is actually installed on the system where make runs.

Note that this extended search for the shell is limited to the cases where SHELL is set from the Makefile; if it is set in the environment or command line, you are expected to set it to the full pathname of the shell, exactly as things are on Unix.

The effect of the above DOS-specific processing is that a Makefile that says `SHELL = /bin/sh' (as many Unix makefiles do), will work on MS-DOS unaltered if you have e.g. `sh.exe' installed in some directory along your PATH.

Unlike most variables, the variable SHELL is never set from the environment. This is because the SHELL environment variable is used to specify your personal choice of shell program for interactive use. It would be very bad for personal choices like this to affect the functioning of makefiles. See section Variables from the Environment. However, on MS-DOS and MS-Windows the value of SHELL in the environment is used, since on those systems most users do not set this variable, and therefore it is most likely set specifically to be used by make. On MS-DOS, if the setting of SHELL is not suitable for make, you can set the variable MAKESHELL to the shell that make should use; this will override the value of SHELL.

5.3 Parallel Execution

GNU make knows how to execute several commands at once. Normally, make will execute only one command at a time, waiting for it to finish before executing the next. However, the `-j' or `--jobs' option tells make to execute many commands simultaneously.

On MS-DOS, the `-j' option has no effect, since that system doesn't support multi-processing.

If the `-j' option is followed by an integer, this is the number of commands to execute at once; this is called the number of job slots. If there is nothing looking like an integer after the `-j' option, there is no limit on the number of job slots. The default number of job slots is one, which means serial execution (one thing at a time).

One unpleasant consequence of running several commands simultaneously is that output generated by the commands appears whenever each command sends it, so messages from different commands may be interspersed.

Another problem is that two processes cannot both take input from the same device; so to make sure that only one command tries to take input from the terminal at once, make will invalidate the standard input streams of all but one running command. This means that attempting to read from standard input will usually be a fatal error (a `Broken pipe' signal) for most child processes if there are several.

It is unpredictable which command will have a valid standard input stream (which will come from the terminal, or wherever you redirect the standard input of make). The first command run will always get it first, and the first command started after that one finishes will get it next, and so on.

We will change how this aspect of make works if we find a better alternative. In the mean time, you should not rely on any command using standard input at all if you are using the parallel execution feature; but if you are not using this feature, then standard input works normally in all commands.

Finally, handling recursive make invocations raises issues. For more information on this, see Communicating Options to a Sub-make.

If a command fails (is killed by a signal or exits with a nonzero status), and errors are not ignored for that command (see section Errors in Commands), the remaining command lines to remake the same target will not be run. If a command fails and the `-k' or `--keep-going' option was not given (see section Summary of Options), make aborts execution. If make terminates for any reason (including a signal) with child processes running, it waits for them to finish before actually exiting.

When the system is heavily loaded, you will probably want to run fewer jobs than when it is lightly loaded. You can use the `-l' option to tell make to limit the number of jobs to run at once, based on the load average. The `-l' or `--max-load' option is followed by a floating-point number. For example,

-l 2.5

will not let make start more than one job if the load average is above 2.5. The `-l' option with no following number removes the load limit, if one was given with a previous `-l' option.

More precisely, when make goes to start up a job, and it already has at least one job running, it checks the current load average; if it is not lower than the limit given with `-l', make waits until the load average goes below that limit, or until all the other jobs finish.

By default, there is no load limit.

5.4 Errors in Commands

After each shell command returns, make looks at its exit status. If the command completed successfully, the next command line is executed in a new shell; after the last command line is finished, the rule is finished.

If there is an error (the exit status is nonzero), make gives up on the current rule, and perhaps on all rules.

Sometimes the failure of a certain command does not indicate a problem. For example, you may use the mkdir command to ensure that a directory exists. If the directory already exists, mkdir will report an error, but you probably want make to continue regardless.

To ignore errors in a command line, write a `-' at the beginning of the line's text (after the initial tab). The `-' is discarded before the command is passed to the shell for execution.

For example,

clean: -rm -f *.o

This causes rm to continue even if it is unable to remove a file.

When you run make with the `-i' or `--ignore-errors' flag, errors are ignored in all commands of all rules. A rule in the makefile for the special target .IGNORE has the same effect, if there are no prerequisites. These ways of ignoring errors are obsolete because `-' is more flexible.

When errors are to be ignored, because of either a `-' or the `-i' flag, make treats an error return just like success, except that it prints out a message that tells you the status code the command exited with, and says that the error has been ignored.

When an error happens that make has not been told to ignore, it implies that the current target cannot be correctly remade, and neither can any other that depends on it either directly or indirectly. No further commands will be executed for these targets, since their preconditions have not been achieved.

Normally make gives up immediately in this circumstance, returning a nonzero status. However, if the `-k' or `--keep-going' flag is specified, make continues to consider the other prerequisites of the pending targets, remaking them if necessary, before it gives up and returns nonzero status. For example, after an error in compiling one object file, `make -k' will continue compiling other object files even though it already knows that linking them will be impossible. See section Summary of Options.

The usual behavior assumes that your purpose is to get the specified targets up to date; once make learns that this is impossible, it might as well report the failure immediately. The `-k' option says that the real purpose is to test as many of the changes made in the program as possible, perhaps to find several independent problems so that you can correct them all before the next attempt to compile. This is why Emacs' compile command passes the `-k' flag by default.

Usually when a command fails, if it has changed the target file at all, the file is corrupted and cannot be used--or at least it is not completely updated. Yet the file's time stamp says that it is now up to date, so the next time make runs, it will not try to update that file. The situation is just the same as when the command is killed by a signal; see section 5.5 Interrupting or Killing make. So generally the right thing to do is to delete the target file if the command fails after beginning to change the file. make will do this if .DELETE_ON_ERROR appears as a target. This is almost always what you want make to do, but it is not historical practice; so for compatibility, you must explicitly request it.

5.5 Interrupting or Killing make

If make gets a fatal signal while a command is executing, it may delete the target file that the command was supposed to update. This is done if the target file's last-modification time has changed since make first checked it.

The purpose of deleting the target is to make sure that it is remade from scratch when make is next run. Why is this? Suppose you type Ctrl-c while a compiler is running, and it has begun to write an object file `foo.o'. The Ctrl-c kills the compiler, resulting in an incomplete file whose last-modification time is newer than the source file `foo.c'. But make also receives the Ctrl-c signal and deletes this incomplete file. If make did not do this, the next invocation of make would think that `foo.o' did not require updating--resulting in a strange error message from the linker when it tries to link an object file half of which is missing.

You can prevent the deletion of a target file in this way by making the special target .PRECIOUS depend on it. Before remaking a target, make checks to see whether it appears on the prerequisites of .PRECIOUS, and thereby decides whether the target should be deleted if a signal happens. Some reasons why you might do this are that the target is updated in some atomic fashion, or exists only to record a modification-time (its contents do not matter), or must exist at all times to prevent other sorts of trouble.

5.6 Recursive Use of make

Recursive use of make means using make as a command in a makefile. This technique is useful when you want separate makefiles for various subsystems that compose a larger system. For example, suppose you have a subdirectory `subdir' which has its own makefile, and you would like the containing directory's makefile to run make on the subdirectory. You can do it by writing this:

subsystem: cd subdir && $(MAKE)

or, equivalently, this (see section Summary of Options):

subsystem: $(MAKE) -C subdir

You can write recursive make commands just by copying this example, but there are many things to know about how they work and why, and about how the sub-make relates to the top-level make. You may also find it useful to declare targets that invoke recursive make commands as `.PHONY' (for more discussion on when this is useful, see 4.6 Phony Targets).

For your convenience, GNU make sets the variable CURDIR to the pathname of the current working directory for you. If -C is in effect, it will contain the path of the new directory, not the original. The value has the same precedence it would have if it were set in the makefile (by default, an environment variable CURDIR will not override this value). Note that setting this variable has no effect on the operation of make

5.6.1 How the MAKE Variable Works		The special effects of using `$(MAKE)'.
5.6.2 Communicating Variables to a Sub-make		How to communicate variables to a sub-make.
5.6.3 Communicating Options to a Sub-make		How to communicate options to a sub-make.
5.6.4 The `--print-directory' Option		How the `-w' or `--print-directory' option helps debug use of recursive make commands.

5.6.1 How the MAKE Variable Works

Recursive make commands should always use the variable MAKE, not the explicit command name `make', as shown here:

subsystem: cd subdir && $(MAKE)

The value of this variable is the file name with which make was invoked. If this file name was `/bin/make', then the command executed is `cd subdir && /bin/make'. If you use a special version of make to run the top-level makefile, the same special version will be executed for recursive invocations.

As a special feature, using the variable MAKE in the commands of a rule alters the effects of the `-t' (`--touch'), `-n' (`--just-print'), or `-q' (`--question') option. Using the MAKE variable has the same effect as using a `+' character at the beginning of the command line. See section Instead of Executing the Commands.

Consider the command `make -t' in the above example. (The `-t' option marks targets as up to date without actually running any commands; see 9.3 Instead of Executing the Commands.) Following the usual definition of `-t', a `make -t' command in the example would create a file named `subsystem' and do nothing else. What you really want it to do is run `cd subdir && make -t'; but that would require executing the command, and `-t' says not to execute commands.

The special feature makes this do what you want: whenever a command line of a rule contains the variable MAKE, the flags `-t', `-n' and `-q' do not apply to that line. Command lines containing MAKE are executed normally despite the presence of a flag that causes most commands not to be run. The usual MAKEFLAGS mechanism passes the flags to the sub-make (see section Communicating Options to a Sub-make), so your request to touch the files, or print the commands, is propagated to the subsystem.

5.6.2 Communicating Variables to a Sub-make

Variable values of the top-level make can be passed to the sub-make through the environment by explicit request. These variables are defined in the sub-make as defaults, but do not override what is specified in the makefile used by the sub-make makefile unless you use the `-e' switch (see section Summary of Options).

To pass down, or export, a variable, make adds the variable and its value to the environment for running each command. The sub-make, in turn, uses the environment to initialize its table of variable values. See section Variables from the Environment.

Except by explicit request, make exports a variable only if it is either defined in the environment initially or set on the command line, and if its name consists only of letters, numbers, and underscores. Some shells cannot cope with environment variable names consisting of characters other than letters, numbers, and underscores.

The special variables SHELL and MAKEFLAGS are always exported (unless you unexport them). MAKEFILES is exported if you set it to anything.

make automatically passes down variable values that were defined on the command line, by putting them in the MAKEFLAGS variable. See section 5.6.3 Communicating Options to a Sub-make.

Variables are not normally passed down if they were created by default by make (see section Variables Used by Implicit Rules). The sub-make will define these for itself.

If you want to export specific variables to a sub-make, use the export directive, like this:

export variable ...

If you want to prevent a variable from being exported, use the unexport directive, like this:

unexport variable ...

In both of these forms, the arguments to export and unexport are expanded, and so could be variables or functions which expand to a (list of) variable names to be (un)exported.

As a convenience, you can define a variable and export it at the same time by doing:

export variable = value

has the same result as:

variable = value export variable

and

export variable := value

has the same result as:

variable := value export variable

Likewise,

export variable += value

is just like:

variable += value export variable

See section Appending More Text to Variables.

You may notice that the export and unexport directives work in make in the same way they work in the shell, sh.

If you want all variables to be exported by default, you can use export by itself:

export

This tells make that variables which are not explicitly mentioned in an export or unexport directive should be exported. Any variable given in an unexport directive will still not be exported. If you use export by itself to export variables by default, variables whose names contain characters other than alphanumerics and underscores will not be exported unless specifically mentioned in an export directive.

The behavior elicited by an export directive by itself was the default in older versions of GNU make. If your makefiles depend on this behavior and you want to be compatible with old versions of make, you can write a rule for the special target .EXPORT_ALL_VARIABLES instead of using the export directive. This will be ignored by old makes, while the export directive will cause a syntax error.

Likewise, you can use unexport by itself to tell make not to export variables by default. Since this is the default behavior, you would only need to do this if export had been used by itself earlier (in an included makefile, perhaps). You cannot use export and unexport by themselves to have variables exported for some commands and not for others. The last export or unexport directive that appears by itself determines the behavior for the entire run of make.

As a special feature, the variable MAKELEVEL is changed when it is passed down from level to level. This variable's value is a string which is the depth of the level as a decimal number. The value is `0' for the top-level make; `1' for a sub-make, `2' for a sub-sub-make, and so on. The incrementation happens when make sets up the environment for a command.

The main use of MAKELEVEL is to test it in a conditional directive (see section Conditional Parts of Makefiles); this way you can write a makefile that behaves one way if run recursively and another way if run directly by you.

You can use the variable MAKEFILES to cause all sub-make commands to use additional makefiles. The value of MAKEFILES is a whitespace-separated list of file names. This variable, if defined in the outer-level makefile, is passed down through the environment; then it serves as a list of extra makefiles for the sub-make to read before the usual or specified ones. See section The Variable MAKEFILES.

5.6.3 Communicating Options to a Sub-make

Flags such as `-s' and `-k' are passed automatically to the sub-make through the variable MAKEFLAGS. This variable is set up automatically by make to contain the flag letters that make received. Thus, if you do `make -ks' then MAKEFLAGS gets the value `ks'.

As a consequence, every sub-make gets a value for MAKEFLAGS in its environment. In response, it takes the flags from that value and processes them as if they had been given as arguments. See section Summary of Options.

Likewise variables defined on the command line are passed to the sub-make through MAKEFLAGS. Words in the value of MAKEFLAGS that contain `=', make treats as variable definitions just as if they appeared on the command line. See section Overriding Variables.

The options `-C', `-f', `-o', and `-W' are not put into MAKEFLAGS; these options are not passed down.

The `-j' option is a special case (see section Parallel Execution). If you set it to some numeric value `N' and your operating system supports it (most any UNIX system will; others typically won't), the parent make and all the sub-makes will communicate to ensure that there are only `N' jobs running at the same time between them all. Note that any job that is marked recursive (see section Instead of Executing the Commands) doesn't count against the total jobs (otherwise we could get `N' sub-makes running and have no slots left over for any real work!)

If your operating system doesn't support the above communication, then `-j 1' is always put into MAKEFLAGS instead of the value you specified. This is because if the `-j' option were passed down to sub-makes, you would get many more jobs running in parallel than you asked for. If you give `-j' with no numeric argument, meaning to run as many jobs as possible in parallel, this is passed down, since multiple infinities are no more than one.

If you do not want to pass the other flags down, you must change the value of MAKEFLAGS, like this:

subsystem: cd subdir && $(MAKE) MAKEFLAGS=

The command line variable definitions really appear in the variable MAKEOVERRIDES, and MAKEFLAGS contains a reference to this variable. If you do want to pass flags down normally, but don't want to pass down the command line variable definitions, you can reset MAKEOVERRIDES to empty, like this:

MAKEOVERRIDES =

This is not usually useful to do. However, some systems have a small fixed limit on the size of the environment, and putting so much information into the value of MAKEFLAGS can exceed it. If you see the error message `Arg list too long', this may be the problem. (For strict compliance with POSIX.2, changing MAKEOVERRIDES does not affect MAKEFLAGS if the special target `.POSIX' appears in the makefile. You probably do not care about this.)

A similar variable MFLAGS exists also, for historical compatibility. It has the same value as MAKEFLAGS except that it does not contain the command line variable definitions, and it always begins with a hyphen unless it is empty (MAKEFLAGS begins with a hyphen only when it begins with an option that has no single-letter version, such as `--warn-undefined-variables'). MFLAGS was traditionally used explicitly in the recursive make command, like this:

subsystem: cd subdir && $(MAKE) $(MFLAGS)

but now MAKEFLAGS makes this usage redundant. If you want your makefiles to be compatible with old make programs, use this technique; it will work fine with more modern make versions too.

The MAKEFLAGS variable can also be useful if you want to have certain options, such as `-k' (see section Summary of Options), set each time you run make. You simply put a value for MAKEFLAGS in your environment. You can also set MAKEFLAGS in a makefile, to specify additional flags that should also be in effect for that makefile. (Note that you cannot use MFLAGS this way. That variable is set only for compatibility; make does not interpret a value you set for it in any way.)

When make interprets the value of MAKEFLAGS (either from the environment or from a makefile), it first prepends a hyphen if the value does not already begin with one. Then it chops the value into words separated by blanks, and parses these words as if they were options given on the command line (except that `-C', `-f', `-h', `-o', `-W', and their long-named versions are ignored; and there is no error for an invalid option).

If you do put MAKEFLAGS in your environment, you should be sure not to include any options that will drastically affect the actions of make and undermine the purpose of makefiles and of make itself. For instance, the `-t', `-n', and `-q' options, if put in one of these variables, could have disastrous consequences and would certainly have at least surprising and probably annoying effects.

5.6.4 The `--print-directory' Option

If you use several levels of recursive make invocations, the `-w' or `--print-directory' option can make the output a lot easier to understand by showing each directory as make starts processing it and as make finishes processing it. For example, if `make -w' is run in the directory `/u/gnu/make', make will print a line of the form:

make: Entering directory `/u/gnu/make'.

before doing anything else, and a line of the form:

make: Leaving directory `/u/gnu/make'.

when processing is completed.

Normally, you do not need to specify this option because `make' does it for you: `-w' is turned on automatically when you use the `-C' option, and in sub-makes. make will not automatically turn on `-w' if you also use `-s', which says to be silent, or if you use `--no-print-directory' to explicitly disable it.

5.7 Defining Canned Command Sequences

When the same sequence of commands is useful in making various targets, you can define it as a canned sequence with the define directive, and refer to the canned sequence from the rules for those targets. The canned sequence is actually a variable, so the name must not conflict with other variable names.

Here is an example of defining a canned sequence of commands:

define run-yacc yacc $(firstword $^) mv y.tab.c $@ endef

Here run-yacc is the name of the variable being defined; endef marks the end of the definition; the lines in between are the commands. The define directive does not expand variable references and function calls in the canned sequence; the `$' characters, parentheses, variable names, and so on, all become part of the value of the variable you are defining. See section Defining Variables Verbatim, for a complete explanation of define.

The first command in this example runs Yacc on the first prerequisite of whichever rule uses the canned sequence. The output file from Yacc is always named `y.tab.c'. The second command moves the output to the rule's target file name.

To use the canned sequence, substitute the variable into the commands of a rule. You can substitute it like any other variable (see section Basics of Variable References). Because variables defined by define are recursively expanded variables, all the variable references you wrote inside the define are expanded now. For example:

foo.c : foo.y $(run-yacc)

`foo.y' will be substituted for the variable `$^' when it occurs in run-yacc's value, and `foo.c' for `$@'.

This is a realistic example, but this particular one is not needed in practice because make has an implicit rule to figure out these commands based on the file names involved (see section Using Implicit Rules).

In command execution, each line of a canned sequence is treated just as if the line appeared on its own in the rule, preceded by a tab. In particular, make invokes a separate subshell for each line. You can use the special prefix characters that affect command lines (`@', `-', and `+') on each line of a canned sequence. See section Writing the Commands in Rules. For example, using this canned sequence:

define frobnicate @echo "frobnicating target $@" frob-step-1 $< -o $@-step-1 frob-step-2 $@-step-1 -o $@ endef

make will not echo the first line, the echo command. But it will echo the following two command lines.

On the other hand, prefix characters on the command line that refers to a canned sequence apply to every line in the sequence. So the rule:

frob.out: frob.in @$(frobnicate)

does not echo any commands. (See section Command Echoing, for a full explanation of `@'.)

5.8 Using Empty Commands

It is sometimes useful to define commands which do nothing. This is done simply by giving a command that consists of nothing but whitespace. For example:

target: ;

defines an empty command string for `target'. You could also use a line beginning with a tab character to define an empty command string, but this would be confusing because such a line looks empty.

You may be wondering why you would want to define a command string that does nothing. The only reason this is useful is to prevent a target from getting implicit commands (from implicit rules or the .DEFAULT special target; see section 10. Using Implicit Rules and see section Defining Last-Resort Default Rules).

You may be inclined to define empty command strings for targets that are not actual files, but only exist so that their prerequisites can be remade. However, this is not the best way to do that, because the prerequisites may not be remade properly if the target file actually does exist. See section Phony Targets, for a better way to do this.

6. How to Use Variables

A variable is a name defined in a makefile to represent a string of text, called the variable's value. These values are substituted by explicit request into targets, prerequisites, commands, and other parts of the makefile. (In some other versions of make, variables are called macros.)

Variables and functions in all parts of a makefile are expanded when read, except for the shell commands in rules, the right-hand sides of variable definitions using `=', and the bodies of variable definitions using the define directive.

Variables can represent lists of file names, options to pass to compilers, programs to run, directories to look in for source files, directories to write output in, or anything else you can imagine.

A variable name may be any sequence of characters not containing `:', `#', `=', or leading or trailing whitespace. However, variable names containing characters other than letters, numbers, and underscores should be avoided, as they may be given special meanings in the future, and with some shells they cannot be passed through the environment to a sub-make (see section Communicating Variables to a Sub-make).

Variable names are case-sensitive. The names `foo', `FOO', and `Foo' all refer to different variables.

It is traditional to use upper case letters in variable names, but we recommend using lower case letters for variable names that serve internal purposes in the makefile, and reserving upper case for parameters that control implicit rules or for parameters that the user should override with command options (see section Overriding Variables).

A few variables have names that are a single punctuation character or just a few characters. These are the automatic variables, and they have particular specialized uses. See section Automatic Variables.

6.1 Basics of Variable References		How to use the value of a variable.
6.2 The Two Flavors of Variables		Variables come in two flavors.
6.3 Advanced Features for Reference to Variables		Advanced features for referencing a variable.
6.4 How Variables Get Their Values		All the ways variables get their values.
6.5 Setting Variables		How to set a variable in the makefile.
6.6 Appending More Text to Variables		How to append more text to the old value of a variable.
6.7 The override Directive		How to set a variable in the makefile even if the user has set it with a command argument.
6.8 Defining Variables Verbatim		An alternate way to set a variable to a verbatim string.
6.9 Variables from the Environment		Variable values can come from the environment.
6.10 Target-specific Variable Values		Variable values can be defined on a per-target basis.
6.11 Pattern-specific Variable Values		Target-specific variable values can be applied to a group of targets that match a pattern.

6.1 Basics of Variable References

To substitute a variable's value, write a dollar sign followed by the name of the variable in parentheses or braces: either `$(foo)' or `${foo}' is a valid reference to the variable foo. This special significance of `$' is why you must write `$$' to have the effect of a single dollar sign in a file name or command.

Variable references can be used in any context: targets, prerequisites, commands, most directives, and new variable values. Here is an example of a common case, where a variable holds the names of all the object files in a program:

objects = program.o foo.o utils.o program : $(objects) cc -o program $(objects) $(objects) : defs.h

Variable references work by strict textual substitution. Thus, the rule

foo = c prog.o : prog.$(foo) $(foo)$(foo) -$(foo) prog.$(foo)

could be used to compile a C program `prog.c'. Since spaces before the variable value are ignored in variable assignments, the value of foo is precisely `c'. (Don't actually write your makefiles this way!)

A dollar sign followed by a character other than a dollar sign, open-parenthesis or open-brace treats that single character as the variable name. Thus, you could reference the variable x with `$x'. However, this practice is strongly discouraged, except in the case of the automatic variables (see section Automatic Variables).

6.2 The Two Flavors of Variables

There are two ways that a variable in GNU make can have a value; we call them the two flavors of variables. The two flavors are distinguished in how they are defined and in what they do when expanded.

The first flavor of variable is a recursively expanded variable. Variables of this sort are defined by lines using `=' (see section Setting Variables) or by the define directive (see section Defining Variables Verbatim). The value you specify is installed verbatim; if it contains references to other variables, these references are expanded whenever this variable is substituted (in the course of expanding some other string). When this happens, it is called recursive expansion.

For example,

foo = $(bar) bar = $(ugh) ugh = Huh? all:;echo $(foo)

will echo `Huh?': `$(foo)' expands to `$(bar)' which expands to `$(ugh)' which finally expands to `Huh?'.

This flavor of variable is the only sort supported by other versions of make. It has its advantages and its disadvantages. An advantage (most would say) is that:

CFLAGS = $(include_dirs) -O include_dirs = -Ifoo -Ibar

will do what was intended: when `CFLAGS' is expanded in a command, it will expand to `-Ifoo -Ibar -O'. A major disadvantage is that you cannot append something on the end of a variable, as in

CFLAGS = $(CFLAGS) -O

because it will cause an infinite loop in the variable expansion. (Actually make detects the infinite loop and reports an error.)

Another disadvantage is that any functions (see section Functions for Transforming Text) referenced in the definition will be executed every time the variable is expanded. This makes make run slower; worse, it causes the wildcard and shell functions to give unpredictable results because you cannot easily control when they are called, or even how many times.

To avoid all the problems and inconveniences of recursively expanded variables, there is another flavor: simply expanded variables.

Simply expanded variables are defined by lines using `:=' (see section Setting Variables). The value of a simply expanded variable is scanned once and for all, expanding any references to other variables and functions, when the variable is defined. The actual value of the simply expanded variable is the result of expanding the text that you write. It does not contain any references to other variables; it contains their values as of the time this variable was defined. Therefore,

x := foo y := $(x) bar x := later

is equivalent to

y := foo bar x := later

When a simply expanded variable is referenced, its value is substituted verbatim.

Here is a somewhat more complicated example, illustrating the use of `:=' in conjunction with the shell function. (See section The shell Function.) This example also shows use of the variable MAKELEVEL, which is changed when it is passed down from level to level. (See section Communicating Variables to a Sub-make, for information about MAKELEVEL.)

ifeq (0,${MAKELEVEL}) cur-dir := $(shell pwd) whoami := $(shell whoami) host-type := $(shell arch) MAKE := ${MAKE} host-type=${host-type} whoami=${whoami} endif

An advantage of this use of `:=' is that a typical `descend into a directory' command then looks like this:

${subdirs}: ${MAKE} cur-dir=${cur-dir}/$@ -C $@ all

Simply expanded variables generally make complicated makefile programming more predictable because they work like variables in most programming languages. They allow you to redefine a variable using its own value (or its value processed in some way by one of the expansion functions) and to use the expansion functions much more efficiently (see section Functions for Transforming Text).

You can also use them to introduce controlled leading whitespace into variable values. Leading whitespace characters are discarded from your input before substitution of variable references and function calls; this means you can include leading spaces in a variable value by protecting them with variable references, like this:

nullstring := space := $(nullstring) # end of the line

Here the value of the variable space is precisely one space. The comment `# end of the line' is included here just for clarity. Since trailing space characters are not stripped from variable values, just a space at the end of the line would have the same effect (but be rather hard to read). If you put whitespace at the end of a variable value, it is a good idea to put a comment like that at the end of the line to make your intent clear. Conversely, if you do not want any whitespace characters at the end of your variable value, you must remember not to put a random comment on the end of the line after some whitespace, such as this:

dir := /foo/bar # directory to put the frobs in

Here the value of the variable dir is `/foo/bar ' (with four trailing spaces), which was probably not the intention. (Imagine something like `$(dir)/file' with this definition!)

There is another assignment operator for variables, `?='. This is called a conditional variable assignment operator, because it only has an effect if the variable is not yet defined. This statement:

FOO ?= bar

is exactly equivalent to this (see section The origin Function):

ifeq ($(origin FOO), undefined) FOO = bar endif

Note that a variable set to an empty value is still defined, so `?=' will not set that variable.

6.3 Advanced Features for Reference to Variables

This section describes some advanced features you can use to reference variables in more flexible ways.

6.3.1 Substitution References		Referencing a variable with substitutions on the value.
6.3.2 Computed Variable Names		Computing the name of the variable to refer to.

6.3.1 Substitution References

A substitution reference substitutes the value of a variable with alterations that you specify. It has the form `$(var:a=b)' (or `${var:a=b}') and its meaning is to take the value of the variable var, replace every a at the end of a word with b in that value, and substitute the resulting string.

When we say "at the end of a word", we mean that a must appear either followed by whitespace or at the end of the value in order to be replaced; other occurrences of a in the value are unaltered. For example:

foo := a.o b.o c.o bar := $(foo:.o=.c)

sets `bar' to `a.c b.c c.c'. See section Setting Variables.

A substitution reference is actually an abbreviation for use of the patsubst expansion function (see section Functions for String Substitution and Analysis). We provide substitution references as well as patsubst for compatibility with other implementations of make.

Another type of substitution reference lets you use the full power of the patsubst function. It has the same form `$(var:a=b)' described above, except that now a must contain a single `%' character. This case is equivalent to `$(patsubst a,b,$(var))'. See section Functions for String Substitution and Analysis, for a description of the patsubst function.

For example: foo := a.o b.o c.o bar := $(foo:%.o=%.c)

sets `bar' to `a.c b.c c.c'.

6.3.2 Computed Variable Names

Computed variable names are a complicated concept needed only for sophisticated makefile programming. For most purposes you need not consider them, except to know that making a variable with a dollar sign in its name might have strange results. However, if you are the type that wants to understand everything, or you are actually interested in what they do, read on.

Variables may be referenced inside the name of a variable. This is called a computed variable name or a nested variable reference. For example,

x = y y = z a := $($(x))

defines a as `z': the `$(x)' inside `$($(x))' expands to `y', so `$($(x))' expands to `$(y)' which in turn expands to `z'. Here the name of the variable to reference is not stated explicitly; it is computed by expansion of `$(x)'. The reference `$(x)' here is nested within the outer variable reference.

The previous example shows two levels of nesting, but any number of levels is possible. For example, here are three levels:

x = y y = z z = u a := $($($(x)))

Here the innermost `$(x)' expands to `y', so `$($(x))' expands to `$(y)' which in turn expands to `z'; now we have `$(z)', which becomes `u'.

References to recursively-expanded variables within a variable name are reexpanded in the usual fashion. For example:

x = $(y) y = z z = Hello a := $($(x))

defines a as `Hello': `$($(x))' becomes `$($(y))' which becomes `$(z)' which becomes `Hello'.

Nested variable references can also contain modified references and function invocations (see section Functions for Transforming Text), just like any other reference. For example, using the subst function (see section Functions for String Substitution and Analysis):

x = variable1 variable2 := Hello y = $(subst 1,2,$(x)) z = y a := $($($(z)))

eventually defines a as `Hello'. It is doubtful that anyone would ever want to write a nested reference as convoluted as this one, but it works: `$($($(z)))' expands to `$($(y))' which becomes `$($(subst 1,2,$(x)))'. This gets the value `variable1' from x and changes it by substitution to `variable2', so that the entire string becomes `$(variable2)', a simple variable reference whose value is `Hello'.

A computed variable name need not consist entirely of a single variable reference. It can contain several variable references, as well as some invariant text. For example,

a_dirs := dira dirb 1_dirs := dir1 dir2 a_files := filea fileb 1_files := file1 file2 ifeq "$(use_a)" "yes" a1 := a else a1 := 1 endif ifeq "$(use_dirs)" "yes" df := dirs else df := files endif dirs := $($(a1)_$(df))

will give dirs the same value as a_dirs, 1_dirs, a_files or 1_files depending on the settings of use_a and use_dirs.

Computed variable names can also be used in substitution references:

a_objects := a.o b.o c.o 1_objects := 1.o 2.o 3.o sources := $($(a1)_objects:.o=.c)

defines sources as either `a.c b.c c.c' or `1.c 2.c 3.c', depending on the value of a1.

The only restriction on this sort of use of nested variable references is that they cannot specify part of the name of a function to be called. This is because the test for a recognized function name is done before the expansion of nested references. For example,

ifdef do_sort func := sort else func := strip endif bar := a d b g q c foo := $($(func) $(bar))

attempts to give `foo' the value of the variable `sort a d b g q c' or `strip a d b g q c', rather than giving `a d b g q c' as the argument to either the sort or the strip function. This restriction could be removed in the future if that change is shown to be a good idea.

You can also use computed variable names in the left-hand side of a variable assignment, or in a define directive, as in:

dir = foo $(dir)_sources := $(wildcard $(dir)/*.c) define $(dir)_print lpr $($(dir)_sources) endef

This example defines the variables `dir', `foo_sources', and `foo_print'.

Note that nested variable references are quite different from recursively expanded variables (see section The Two Flavors of Variables), though both are used together in complex ways when doing makefile programming.

6.4 How Variables Get Their Values

Variables can get values in several different ways:

• You can specify an overriding value when you run make. See section Overriding Variables.

• You can specify a value in the makefile, either with an assignment (see section Setting Variables) or with a verbatim definition (see section Defining Variables Verbatim).

• Variables in the environment become make variables. See section Variables from the Environment.

• Several automatic variables are given new values for each rule. Each of these has a single conventional use. See section Automatic Variables.

• Several variables have constant initial values. See section Variables Used by Implicit Rules.

6.5 Setting Variables

To set a variable from the makefile, write a line starting with the variable name followed by `=' or `:='. Whatever follows the `=' or `:=' on the line becomes the value. For example,

objects = main.o foo.o bar.o utils.o

defines a variable named objects. Whitespace around the variable name and immediately after the `=' is ignored.

Variables defined with `=' are recursively expanded variables. Variables defined with `:=' are simply expanded variables; these definitions can contain variable references which will be expanded before the definition is made. See section The Two Flavors of Variables.

The variable name may contain function and variable references, which are expanded when the line is read to find the actual variable name to use.

There is no limit on the length of the value of a variable except the amount of swapping space on the computer. When a variable definition is long, it is a good idea to break it into several lines by inserting backslash-newline at convenient places in the definition. This will not affect the functioning of make, but it will make the makefile easier to read.

Most variable names are considered to have the empty string as a value if you have never set them. Several variables have built-in initial values that are not empty, but you can set them in the usual ways (see section Variables Used by Implicit Rules). Several special variables are set automatically to a new value for each rule; these are called the automatic variables (see section Automatic Variables).

If you'd like a variable to be set to a value only if it's not already set, then you can use the shorthand operator `?=' instead of `='. These two settings of the variable `FOO' are identical (see section The origin Function):

FOO ?= bar

and

ifeq ($(origin FOO), undefined) FOO = bar endif

6.6 Appending More Text to Variables

Often it is useful to add more text to the value of a variable already defined. You do this with a line containing `+=', like this:

objects += another.o

This takes the value of the variable objects, and adds the text `another.o' to it (preceded by a single space). Thus:

objects = main.o foo.o bar.o utils.o objects += another.o

sets objects to `main.o foo.o bar.o utils.o another.o'.

Using `+=' is similar to:

objects = main.o foo.o bar.o utils.o objects := $(objects) another.o

but differs in ways that become important when you use more complex values.

When the variable in question has not been defined before, `+=' acts just like normal `=': it defines a recursively-expanded variable. However, when there is a previous definition, exactly what `+=' does depends on what flavor of variable you defined originally. See section The Two Flavors of Variables, for an explanation of the two flavors of variables.

When you add to a variable's value with `+=', make acts essentially as if you had included the extra text in the initial definition of the variable. If you defined it first with `:=', making it a simply-expanded variable, `+=' adds to that simply-expanded definition, and expands the new text before appending it to the old value just as `:=' does (see section Setting Variables, for a full explanation of `:='). In fact,

variable := value variable += more

is exactly equivalent to:

variable := value variable := $(variable) more

On the other hand, when you use `+=' with a variable that you defined first to be recursively-expanded using plain `=', make does something a bit different. Recall that when you define a recursively-expanded variable, make does not expand the value you set for variable and function references immediately. Instead it stores the text verbatim, and saves these variable and function references to be expanded later, when you refer to the new variable (see section The Two Flavors of Variables). When you use `+=' on a recursively-expanded variable, it is this unexpanded text to which make appends the new text you specify.

variable = value variable += more

is roughly equivalent to:

temp = value variable = $(temp) more

except that of course it never defines a variable called temp. The importance of this comes when the variable's old value contains variable references. Take this common example:

CFLAGS = $(includes) -O ... CFLAGS += -pg # enable profiling

The first line defines the CFLAGS variable with a reference to another variable, includes. (CFLAGS is used by the rules for C compilation; see section Catalogue of Implicit Rules.) Using `=' for the definition makes CFLAGS a recursively-expanded variable, meaning `$(includes) -O' is not expanded when make processes the definition of CFLAGS. Thus, includes need not be defined yet for its value to take effect. It only has to be defined before any reference to CFLAGS. If we tried to append to the value of CFLAGS without using `+=', we might do it like this:

CFLAGS := $(CFLAGS) -pg # enable profiling

This is pretty close, but not quite what we want. Using `:=' redefines CFLAGS as a simply-expanded variable; this means make expands the text `$(CFLAGS) -pg' before setting the variable. If includes is not yet defined, we get ` -O -pg', and a later definition of includes will have no effect. Conversely, by using `+=' we set CFLAGS to the unexpanded value `$(includes) -O -pg'. Thus we preserve the reference to includes, so if that variable gets defined at any later point, a reference like `$(CFLAGS)' still uses its value.

6.7 The override Directive

If a variable has been set with a command argument (see section Overriding Variables), then ordinary assignments in the makefile are ignored. If you want to set the variable in the makefile even though it was set with a command argument, you can use an override directive, which is a line that looks like this:

override variable = value

or

override variable := value

To append more text to a variable defined on the command line, use:

override variable += more text

See section Appending More Text to Variables.

The override directive was not invented for escalation in the war between makefiles and command arguments. It was invented so you can alter and add to values that the user specifies with command arguments.

For example, suppose you always want the `-g' switch when you run the C compiler, but you would like to allow the user to specify the other switches with a command argument just as usual. You could use this override directive:

override CFLAGS += -g

You can also use override directives with define directives. This is done as you might expect:

override define foo bar endef

See section Defining Variables Verbatim.

6.8 Defining Variables Verbatim

Another way to set the value of a variable is to use the define directive. This directive has an unusual syntax which allows newline characters to be included in the value, which is convenient for defining both canned sequences of commands (see section Defining Canned Command Sequences), and also sections of makefile syntax to use with eval (see section 8.8 The eval Function).

The define directive is followed on the same line by the name of the variable and nothing more. The value to give the variable appears on the following lines. The end of the value is marked by a line containing just the word endef. Aside from this difference in syntax, define works just like `=': it creates a recursively-expanded variable (see section The Two Flavors of Variables). The variable name may contain function and variable references, which are expanded when the directive is read to find the actual variable name to use.

You may nest define directives: make will keep track of nested directives and report an error if they are not all properly closed with endef. Note that lines beginning with tab characters are considered part of a command script, so any define or endef strings appearing on such a line will not be considered make operators.

define two-lines echo foo echo $(bar) endef

The value in an ordinary assignment cannot contain a newline; but the newlines that separate the lines of the value in a define become part of the variable's value (except for the final newline which precedes the endef and is not considered part of the value).

When used in a command script, the previous example is functionally equivalent to this:

two-lines = echo foo; echo $(bar)

since two commands separated by semicolon behave much like two separate shell commands. However, note that using two separate lines means make will invoke the shell twice, running an independent subshell for each line. See section Command Execution.

If you want variable definitions made with define to take precedence over command-line variable definitions, you can use the override directive together with define:

override define two-lines foo $(bar) endef

See section The override Directive.

6.9 Variables from the Environment

Variables in make can come from the environment in which make is run. Every environment variable that make sees when it starts up is transformed into a make variable with the same name and value. But an explicit assignment in the makefile, or with a command argument, overrides the environment. (If the `-e' flag is specified, then values from the environment override assignments in the makefile. See section Summary of Options. But this is not recommended practice.)

Thus, by setting the variable CFLAGS in your environment, you can cause all C compilations in most makefiles to use the compiler switches you prefer. This is safe for variables with standard or conventional meanings because you know that no makefile will use them for other things. (But this is not totally reliable; some makefiles set CFLAGS explicitly and therefore are not affected by the value in the environment.)

When make is invoked recursively, variables defined in the outer invocation can be passed to inner invocations through the environment (see section Recursive Use of make). By default, only variables that came from the environment or the command line are passed to recursive invocations. You can use the export directive to pass other variables. See section Communicating Variables to a Sub-make, for full details.

Other use of variables from the environment is not recommended. It is not wise for makefiles to depend for their functioning on environment variables set up outside their control, since this would cause different users to get different results from the same makefile. This is against the whole purpose of most makefiles.

Such problems would be especially likely with the variable SHELL, which is normally present in the environment to specify the user's choice of interactive shell. It would be very undesirable for this choice to affect make. So make ignores the environment value of SHELL (except on MS-DOS and MS-Windows, where SHELL is usually not set. See section Special handling of SHELL on MS-DOS.)

6.10 Target-specific Variable Values

Variable values in make are usually global; that is, they are the same regardless of where they are evaluated (unless they're reset, of course). One exception to that is automatic variables (see section Automatic Variables).

The other exception is target-specific variable values. This feature allows you to define different values for the same variable, based on the target that make is currently building. As with automatic variables, these values are only available within the context of a target's command script (and in other target-specific assignments).

Set a target-specific variable value like this:

target ... : variable-assignment

or like this:

target ... : override variable-assignment

Multiple target values create a target-specific variable value for each member of the target list individually.

The variable-assignment can be any valid form of assignment; recursive (`='), static (`:='), appending (`+='), or conditional (`?='). All variables that appear within the variable-assignment are evaluated within the context of the target: thus, any previously-defined target-specific variable values will be in effect. Note that this variable is actually distinct from any "global" value: the two variables do not have to have the same flavor (recursive vs. static).

Target-specific variables have the same priority as any other makefile variable. Variables provided on the command-line (and in the environment if the `-e' option is in force) will take precedence. Specifying the override directive will allow the target-specific variable value to be preferred.

There is one more special feature of target-specific variables: when you define a target-specific variable, that variable value is also in effect for all prerequisites of this target (unless those prerequisites override it with their own target-specific variable value). So, for example, a statement like this:

prog : CFLAGS = -g prog : prog.o foo.o bar.o

will set CFLAGS to `-g' in the command script for `prog', but it will also set CFLAGS to `-g' in the command scripts that create `prog.o', `foo.o', and `bar.o', and any command scripts which create their prerequisites.

6.11 Pattern-specific Variable Values

In addition to target-specific variable values (see section Target-specific Variable Values), GNU make supports pattern-specific variable values. In this form, a variable is defined for any target that matches the pattern specified. Variables defined in this way are searched after any target-specific variables defined explicitly for that target, and before target-specific variables defined for the parent target.

Set a pattern-specific variable value like this:

pattern ... : variable-assignment

or like this:

pattern ... : override variable-assignment

where pattern is a %-pattern. As with target-specific variable values, multiple pattern values create a pattern-specific variable value for each pattern individually. The variable-assignment can be any valid form of assignment. Any command-line variable setting will take precedence, unless override is specified.

For example:

%.o : CFLAGS = -O

will assign CFLAGS the value of `-O' for all targets matching the pattern %.o.

7. Conditional Parts of Makefiles

A conditional causes part of a makefile to be obeyed or ignored depending on the values of variables. Conditionals can compare the value of one variable to another, or the value of a variable to a constant string. Conditionals control what make actually "sees" in the makefile, so they cannot be used to control shell commands at the time of execution.

7.1 Example of a Conditional		Example of a conditional
7.2 Syntax of Conditionals		The syntax of conditionals.
7.3 Conditionals that Test Flags		Conditionals that test flags.

7.1 Example of a Conditional

The following example of a conditional tells make to use one set of libraries if the CC variable is `gcc', and a different set of libraries otherwise. It works by controlling which of two command lines will be used as the command for a rule. The result is that `CC=gcc' as an argument to make changes not only which compiler is used but also which libraries are linked.

libs_for_gcc = -lgnu normal_libs = foo: $(objects) ifeq ($(CC),gcc) $(CC) -o foo $(objects) $(libs_for_gcc) else $(CC) -o foo $(objects) $(normal_libs) endif

This conditional uses three directives: one ifeq, one else and one endif.

The ifeq directive begins the conditional, and specifies the condition. It contains two arguments, separated by a comma and surrounded by parentheses. Variable substitution is performed on both arguments and then they are compared. The lines of the makefile following the ifeq are obeyed if the two arguments match; otherwise they are ignored.

The else directive causes the following lines to be obeyed if the previous conditional failed. In the example above, this means that the second alternative linking command is used whenever the first alternative is not used. It is optional to have an else in a conditional.

The endif directive ends the conditional. Every conditional must end with an endif. Unconditional makefile text follows.

As this example illustrates, conditionals work at the textual level: the lines of the conditional are treated as part of the makefile, or ignored, according to the condition. This is why the larger syntactic units of the makefile, such as rules, may cross the beginning or the end of the conditional.

When the variable CC has the value `gcc', the above example has this effect:

foo: $(objects) $(CC) -o foo $(objects) $(libs_for_gcc)

When the variable CC has any other value, the effect is this:

foo: $(objects) $(CC) -o foo $(objects) $(normal_libs)

Equivalent results can be obtained in another way by conditionalizing a variable assignment and then using the variable unconditionally:

libs_for_gcc = -lgnu normal_libs = ifeq ($(CC),gcc) libs=$(libs_for_gcc) else libs=$(normal_libs) endif foo: $(objects) $(CC) -o foo $(objects) $(libs)

7.2 Syntax of Conditionals

The syntax of a simple conditional with no else is as follows:

conditional-directive text-if-true endif

The text-if-true may be any lines of text, to be considered as part of the makefile if the condition is true. If the condition is false, no text is used instead.

The syntax of a complex conditional is as follows:

conditional-directive text-if-true else text-if-false endif

If the condition is true, text-if-true is used; otherwise, text-if-false is used instead. The text-if-false can be any number of lines of text.

The syntax of the conditional-directive is the same whether the conditional is simple or complex. There are four different directives that test different conditions. Here is a table of them:

ifeq (arg1, arg2)

ifeq 'arg1' 'arg2'

ifeq "arg1" "arg2"

ifeq "arg1" 'arg2'

ifeq 'arg1' "arg2"

Expand all variable references in arg1 and arg2 and compare them. If they are identical, the text-if-true is effective; otherwise, the text-if-false, if any, is effective.

Often you want to test if a variable has a non-empty value. When the value results from complex expansions of variables and functions, expansions you would consider empty may actually contain whitespace characters and thus are not seen as empty. However, you can use the strip function (see section 8.2 Functions for String Substitution and Analysis) to avoid interpreting whitespace as a non-empty value. For example:

ifeq ($(strip $(foo)),) text-if-empty endif

will evaluate text-if-empty even if the expansion of $(foo) contains whitespace characters.

ifneq (arg1, arg2)

ifneq 'arg1' 'arg2'

ifneq "arg1" "arg2"

ifneq "arg1" 'arg2'

ifneq 'arg1' "arg2"

Expand all variable references in arg1 and arg2 and compare them. If they are different, the text-if-true is effective; otherwise, the text-if-false, if any, is effective.

ifdef variable-name

If the variable variable-name has a non-empty value, the text-if-true is effective; otherwise, the text-if-false, if any, is effective. Variables that have never been defined have an empty value. The variable variable-name is itself expanded, so it could be a variable or function that expands to the name of a variable.

Note that ifdef only tests whether a variable has a value. It does not expand the variable to see if that value is nonempty. Consequently, tests using ifdef return true for all definitions except those like foo =. To test for an empty value, use ifeq ($(foo),). For example,

bar = foo = $(bar) ifdef foo frobozz = yes else frobozz = no endif

sets `frobozz' to `yes', while:

foo = ifdef foo frobozz = yes else frobozz = no endif

sets `frobozz' to `no'.

ifndef variable-name

If the variable variable-name has an empty value, the text-if-true is effective; otherwise, the text-if-false, if any, is effective.

Extra spaces are allowed and ignored at the beginning of the conditional directive line, but a tab is not allowed. (If the line begins with a tab, it will be considered a command for a rule.) Aside from this, extra spaces or tabs may be inserted with no effect anywhere except within the directive name or within an argument. A comment starting with `#' may appear at the end of the line.

The other two directives that play a part in a conditional are else and endif. Each of these directives is written as one word, with no arguments. Extra spaces are allowed and ignored at the beginning of the line, and spaces or tabs at the end. A comment starting with `#' may appear at the end of the line.

Conditionals affect which lines of the makefile make uses. If the condition is true, make reads the lines of the text-if-true as part of the makefile; if the condition is false, make ignores those lines completely. It follows that syntactic units of the makefile, such as rules, may safely be split across the beginning or the end of the conditional.

make evaluates conditionals when it reads a makefile. Consequently, you cannot use automatic variables in the tests of conditionals because they are not defined until commands are run (see section Automatic Variables).

To prevent intolerable confusion, it is not permitted to start a conditional in one makefile and end it in another. However, you may write an include directive within a conditional, provided you do not attempt to terminate the conditional inside the included file.

7.3 Conditionals that Test Flags

You can write a conditional that tests make command flags such as `-t' by using the variable MAKEFLAGS together with the findstring function (see section Functions for String Substitution and Analysis). This is useful when touch is not enough to make a file appear up to date.

The findstring function determines whether one string appears as a substring of another. If you want to test for the `-t' flag, use `t' as the first string and the value of MAKEFLAGS as the other.

For example, here is how to arrange to use `ranlib -t' to finish marking an archive file up to date:

archive.a: ... ifneq (,$(findstring t,$(MAKEFLAGS))) +touch archive.a +ranlib -t archive.a else ranlib archive.a endif

The `+' prefix marks those command lines as "recursive" so that they will be executed despite use of the `-t' flag. See section Recursive Use of make.

8. Functions for Transforming Text

Functions allow you to do text processing in the makefile to compute the files to operate on or the commands to use. You use a function in a function call, where you give the name of the function and some text (the arguments) for the function to operate on. The result of the function's processing is substituted into the makefile at the point of the call, just as a variable might be substituted.

8.1 Function Call Syntax		How to write a function call.
8.2 Functions for String Substitution and Analysis		General-purpose text manipulation functions.
8.3 Functions for File Names		Functions for manipulating file names.
8.4 The foreach Function		Repeat some text with controlled variation.
8.5 The if Function		Conditionally expand a value.
8.6 The call Function		Expand a user-defined function.
8.7 The value Function		Return the un-expanded value of a variable.
8.8 The eval Function		Evaluate the arguments as makefile syntax.
8.9 The origin Function		Find where a variable got its value.
8.10 The shell Function		Substitute the output of a shell command.
8.11 Functions That Control Make		Functions that control how make runs.

8.1 Function Call Syntax

A function call resembles a variable reference. It looks like this:

$(function arguments)

or like this:

${function arguments}

Here function is a function name; one of a short list of names that are part of make. You can also essentially create your own functions by using the call builtin function.

The arguments are the arguments of the function. They are separated from the function name by one or more spaces or tabs, and if there is more than one argument, then they are separated by commas. Such whitespace and commas are not part of an argument's value. The delimiters which you use to surround the function call, whether parentheses or braces, can appear in an argument only in matching pairs; the other kind of delimiters may appear singly. If the arguments themselves contain other function calls or variable references, it is wisest to use the same kind of delimiters for all the references; write `$(subst a,b,$(x))', not `$(subst a,b,${x})'. This is because it is clearer, and because only one type of delimiter is matched to find the end of the reference.

The text written for each argument is processed by substitution of variables and function calls to produce the argument value, which is the text on which the function acts. The substitution is done in the order in which the arguments appear.

Commas and unmatched parentheses or braces cannot appear in the text of an argument as written; leading spaces cannot appear in the text of the first argument as written. These characters can be put into the argument value by variable substitution. First define variables comma and space whose values are isolated comma and space characters, then substitute these variables where such characters are wanted, like this:

comma:= , empty:= space:= $(empty) $(empty) foo:= a b c bar:= $(subst $(space),$(comma),$(foo)) # bar is now `a,b,c'.

Here the subst function replaces each space with a comma, through the value of foo, and substitutes the result.

8.2 Functions for String Substitution and Analysis

Here are some functions that operate on strings:

$(subst from,to,text)

Performs a textual replacement on the text text: each occurrence of from is replaced by to. The result is substituted for the function call. For example,

$(subst ee,EE,feet on the street)

substitutes the string `fEEt on the strEEt'.

$(patsubst pattern,replacement,text)

Finds whitespace-separated words in text that match pattern and replaces them with replacement. Here pattern may contain a `%' which acts as a wildcard, matching any number of any characters within a word. If replacement also contains a `%', the `%' is replaced by the text that matched the `%' in pattern. Only the first `%' in the pattern and replacement is treated this way; any subsequent `%' is unchanged.

`%' characters in patsubst function invocations can be quoted with preceding backslashes (`\'). Backslashes that would otherwise quote `%' characters can be quoted with more backslashes. Backslashes that quote `%' characters or other backslashes are removed from the pattern before it is compared file names or has a stem substituted into it. Backslashes that are not in danger of quoting `%' characters go unmolested. For example, the pattern `the\%weird\\%pattern\\' has `the%weird\' preceding the operative `%' character, and `pattern\\' following it. The final two backslashes are left alone because they cannot affect any `%' character.

Whitespace between words is folded into single space characters; leading and trailing whitespace is discarded.

For example,

$(patsubst %.c,%.o,x.c.c bar.c)

produces the value `x.c.o bar.o'.

Substitution references (see section Substitution References) are a simpler way to get the effect of the patsubst function:

$(var:pattern=replacement)

is equivalent to

$(patsubst pattern,replacement,$(var))

The second shorthand simplifies one of the most common uses of patsubst: replacing the suffix at the end of file names.

$(var:suffix=replacement)

is equivalent to

$(patsubst %suffix,%replacement,$(var))

For example, you might have a list of object files:

objects = foo.o bar.o baz.o

To get the list of corresponding source files, you could simply write:

$(objects:.o=.c)

instead of using the general form:

$(patsubst %.o,%.c,$(objects))

$(strip string)

Removes leading and trailing whitespace from string and replaces each internal sequence of one or more whitespace characters with a single space. Thus, `$(strip a b c )' results in `a b c'.

The function strip can be very useful when used in conjunction with conditionals. When comparing something with the empty string `' using ifeq or ifneq, you usually want a string of just whitespace to match the empty string (see section 7. Conditional Parts of Makefiles).

Thus, the following may fail to have the desired results:

.PHONY: all ifneq "$(needs_made)" "" all: $(needs_made) else all:;@echo 'Nothing to make!' endif

Replacing the variable reference `$(needs_made)' with the function call `$(strip $(needs_made))' in the ifneq directive would make it more robust.

$(findstring find,in)

Searches in for an occurrence of find. If it occurs, the value is find; otherwise, the value is empty. You can use this function in a conditional to test for the presence of a specific substring in a given string. Thus, the two examples,

$(findstring a,a b c) $(findstring a,b c)

produce the values `a' and `' (the empty string), respectively. See section 7.3 Conditionals that Test Flags, for a practical application of findstring.

$(filter pattern...,text)

Returns all whitespace-separated words in text that do match any of the pattern words, removing any words that do not match. The patterns are written using `%', just like the patterns used in the patsubst function above.

The filter function can be used to separate out different types of strings (such as file names) in a variable. For example:

sources := foo.c bar.c baz.s ugh.h foo: $(sources) cc $(filter %.c %.s,$(sources)) -o foo

says that `foo' depends of `foo.c', `bar.c', `baz.s' and `ugh.h' but only `foo.c', `bar.c' and `baz.s' should be specified in the command to the compiler.

$(filter-out pattern...,text)

Returns all whitespace-separated words in text that do not match any of the pattern words, removing the words that do match one or more. This is the exact opposite of the filter function.

For example, given:

objects=main1.o foo.o main2.o bar.o mains=main1.o main2.o

the following generates a list which contains all the object files not in `mains':

$(filter-out $(mains),$(objects))

$(sort list)

Sorts the words of list in lexical order, removing duplicate words. The output is a list of words separated by single spaces. Thus,

$(sort foo bar lose)

returns the value `bar foo lose'.

Incidentally, since sort removes duplicate words, you can use it for this purpose even if you don't care about the sort order.

$(word n,text)

Returns the nth word of text. The legitimate values of n start from 1. If n is bigger than the number of words in text, the value is empty. For example,

$(word 2, foo bar baz)

returns `bar'.

$(wordlist s,e,text)

Returns the list of words in text starting with word s and ending with word e (inclusive). The legitimate values of s and e start from 1. If s is bigger than the number of words in text, the value is empty. If e is bigger than the number of words in text, words up to the end of text are returned. If s is greater than e, nothing is returned. For example,

$(wordlist 2, 3, foo bar baz)

returns `bar baz'.

$(words text)

Returns the number of words in text. Thus, the last word of text is $(word $(words text),text).

$(firstword names...)

The argument names is regarded as a series of names, separated by whitespace. The value is the first name in the series. The rest of the names are ignored.

For example,

$(firstword foo bar)

produces the result `foo'. Although $(firstword text) is the same as $(word 1,text), the firstword function is retained for its simplicity.